While size is not the only indicator of a body's astrological influence or importance, it is certainly a primary one. Here are some of the larger extra bodies or minor planets as well as stars that you should consider including into your chart interpretation as well as some of their basic characteristics or identifying features...

SUN and MOON: the two Lights or Luminaries, one masculine or positive and the other feminine or negative. which form the most basic personal qualities of the native
MERCURY, VENUS, MARS: the fastest of the traditional planets or "wandering stars" known since antiquity and forming the basic everyday working qualities of the native
JUPITER and SATURN: the slower of the traditional planets which begin the collective process and introduce the native to the larger issues of life such as spirituality and government
URANUS, NEPTUNE, and PLUTO: the modern or transcendental planets discovered since 1781 and forming the more collective as well as individual traits of the native
ASCENDANT and MIDHEAVEN: the two "angles" which along with their opposites form the basic structure or framework of the chart indicating the most personal or observable features of the native which he or she is learning to incorporate in this lifetime.
The Ascendant is actually the Earth's influence as seen from the individual's time and place of birth, whereas the Midheaven is the planet's inner sun which projects itself as the highest point in the sky or celestial sphere along the ecliptic -also as seen from the native's time and place of birth .
Note: These 12 factors are absolutely essential and form the main structure of the chart

Extra non-planetary factors include: the North Node(where the Moon's orbit intersects the ecliptic), the Vertex, and the Part of Fortune

Our cosmos does not consist soley of the Earth(Asc) and Sky or Heaven center(MC), or Sun and Moon and Planets. What about the stars which up untill recently modern astrology has almost entirely omitted? What about the comets, asteroids, nebulae, galaxies, and other objects both within and without our solar system? Certainly they too have their place in the scheme of things -including in our charts and personal lives. While many of these bodies may not be as potent as the basic twelve factors, they certainly add to or refine the understanding or interpretation of a chart.

The star Sirius, for example, is the brightest star in the heavens, and had enormous importance in ancient times and cultures, so why should it not have importance today? Granted, it's very slow-moving and therefore is more of a collective influence, but it's an influence none the less. The influences of the four largest or main asteroids: Ceres, Vesta, Pallas, and Juno, have been around for a while and their influences fairly well noted. Chiron, Pholus, Nessus, Asbolus, and Chariklo are newer yet their influences are slowly unraveling. Newest of all are the dwarf planets, TNO's, and scattered objects discovered since 1992 which are revolutionizing our understanding of the vast collective society, world, or universe we live in. Bodies like Eris, Quaoar, Varuna, and Sedna, while smaller than regular planets, all have something to say or add to concerning ourselves and our reality.

Below are groups or listings of extra bodies you should be paying attention to as our knowledge of astronomy and astrology greatly expand in this exciting 21st century.

Asteroids
CERES: discovered 1801, is the largest(950 km/590 mi) asteroid in a 4.6 year orbit between Mars and Jupiter. Associated with the principle of nurturing and was recently(2006) classified as a dwarf planet.
PALLAS: discovered in 1802, 550 km in size, and associated with wisdom, intelligence and healing
JUNO: discovered in 1804, 320x200 km in size, and associated with love, romance, and marriage
VESTA: discovered in 1807, 530 km in size, and associated with devotion and purity
HYGIEA: discovered in 1849, 431 km in size, and associated with health and medicine

Centaurs
CHIRON: discovered 1977, is the second largest(233 km) but most popular centaur in a 51-year orbit between Saturn and Uranus. Associated with wisdom, healing, teaching, and skill
PHOLUS: discovered in 1992, is 185 km in size, in a 92-year orbit between Saturn and Pluto, and associated with extremism
NESSUS: discovered in 1993, 60 km in size, in a 122-year orbit between Saturn and Pluto, and associated with our deep dark side
ASBOLUS: discovered in 1995, 84 km in size, in a 77-year orbit between Jupiter and Neptune and associated with seership and dark mysticism
HYLONOME: discovered in 1995, 70 km in size, in a 126-year orbit between Uranus and Neptune and associated with strong yearning and idealism
CHARIKLO: discovered in 1997, is the largest cebtaur at 260 km in size, in a 63-year orbit between Saturn and Uranus and associated with transcending daily reality
AMYCUS: discovered in 2002, 76 km in size, in a 127-year orbit between Uranus and Neptune, and associated with progress through adversity or competition

Dwarf Planets or TNO's
VARUNA: discovered in 2000, 700-800 km , with a 281-year orbit, and is associated with raw energy, genius, and strength
IXION: discovered in 2001, 650 km, with a 250-year orbit, and is associated with brashness and impulsivity
QUAOAR: discovered in 2002, 890 km, with a 286-year orbit, and is associated with newness and creativity SEDNA: discovered in 2003, 800-1100 km in size, with a 11,400-year orbit, and is associated with victimization and taboo
MAKEMAKE: discovered in 2004, 750 km, with a 310-year orbit, and is associated with looseness and eccentricity
HAUMEA: discovered in 2004, 1960 x1518 x996 km, with a 285-year orbit, and is associated with purity and simplicity
ORCUS: discovered in 2004, 950 km, with a 245-year orbit, and is associated with hidden wealth or power
ERIS: discovered 2005, 2600 km in size or slightly larger than Pluto, with a 557-year orbit. Associated with the principle of discord or disruption

Stars(24 brightest with magnitude, current position, and associations)
SUN: home star, -26.74, associated with ego or centerdness
SIRIUS: -1.46, 14°Cancer, associated with fame, power, wealth, and danger
CANOPUS: -0.72, 15°Cancer, associated withtravel, fame, wealth, and calamity
RIGEL KENTAURUS: -0.27, 29°Scorpio, associated withoccult learning and honours
ARCTURUS: -0.04, 24°Libra, associated with wealth, fame, and success
VEGA: 0.03, 15°Capricorn, associated with politics, art, and finance
CAPELLA: 0.08, 22°Gemini, associated with learning and powerful associations
RIGEL: 0.12, 17°Gemini, associated with skill and success
PROCYON: 0.38, 25-26°Cancer, associated with success followed by disaster
ACHERNAR: 0.46, 15°Pisces, associated with political and religious success
BETELGEUSE: 0.50, 28-29°Gemini, associated with fortune, fame, and demise
HADAR(AGENA): 0.61, 24°Scorpio, associated with health and success but sadness in love
ACRUX: 0.76, 12°Scorpio, associated with metaphysics and the occult
ALTAIR: 0.77, 1-2°Aquarius, associated with fleeting fame, art, writing, and astrology
ALDEBARAN: 0.85, 9-10° Gemini, and associated with honour, courage, and death
ANTARES: 0.96, 9-10° Sagittarius, associated with passion, battle, and treachery
SPICA: 0.98, 24°Libra, associated with great fortune and fame
POLLUX: 1.14, 23°Cancer, associated with speculation and fall from grace
FOMALHAUT: 1.16, 4°Pisces, associated with lasting honours and occult ability
BECRUX(MIMOSA): 1.25, 11-12°Scorpio, associated with hardship and responsibility
DENEB ADIGE: 1.25, 5°Pisces, associated with intelligence, creativity, writing, and astrology
REGULUS: 1.35, 29°Leo-0°Virgo, associated with nobility, power, and greatness
ADHARA: 1.50, 20-21°Cancer, associated with adulation and adornment
CASTOR: 1.57, 20°Cancer, associated with meteoric rise and downfall and mental pursuits

Galaxies and Other
GALACTIC CENTER: currently located at 27° tropical Sagittarius, and is associated with the womb and our earliest beginnings
ANDROMEDA: larger than our own Milky Way, currently located at 27° of tropical Aries, and is associated with divine intervention
SUPER GALACTIC CENTER: currently located at 2° tropical Libra, and is associated with a deep hunger for unity and wholeness
FOR OTHER DISTANT OBJECTS see here