Humans should be much larger than they are now considering the size of the plane we live on.
At an average height of 5'9" it would take almost a year(336 days) to walk across the Earth's
24,900 mile span near the equator. At that rate, one would have to be over 1900 feet tall
to walk the Earth as we know it in one 24-hour day.
There are legends of giants existing in our ancient past, and some may still be living today. The great "Ages" of our past may simply have been resets by our controllers to reduce both the stature and longetivity of human beings, as well as the rest of our environment, which would make it easier to control us.
According to Hindu mythology, the first humans of the Golden Age(Satyayuga) were a very tall 32 feet and lived a virtually sinless life of some 100,000 years. It was likely during this age that we walked with our divine parents/creators who were even taller and more long-lived than us.
In the Tretayuga(Silver Age) humans were 21 feet tall and lived on average 10,000 years, with a sin rate of 25%. This was the beginning of the fall of man(and perhaps the gods as well) when sin took hold and shortened both the stature and lifespan and mental and spiritual abilities.
In the Dwaparuga(Copper Age) humans were 11 feet tall and lived to be 1000 years with a 50% sin rate. Degeneration of the gods and man continued further shortening or shrinking all aspects of our being.
In the current Kaliyuga(Iron Age) humans average 5.5 ft tall and live about 100 years with a 75% sin rate. This is the darkest period in history where we have lost nearly all our divine abilities and grovel like animals.
Material law tends towards degeneration rather than regeneration and the more we live on this plane, the smaller and shorter and more sinful our lives tend to become. Very few swim against the tide, but those who do(like Krishna and Jesus) are in a position to help the rest of us.
As we come out of the Kaliyuga or midpoint of the 4th root race, our lives(collectively) begin to get better and we become taller and more long-lived. According to Sri Yukteswar, this began to happen around 498 AD or just after the official death of the ancient Roman Empire in 476 AD. The Beast finally died and humanity was given another chance.
Individually, we can embark on the path towards regeneration ourselves, regardless of the Age or time period.
In the Hindu scriptures the Four Ages are given very long periods of 1,728,000 years for Satyayuga, 1,296,000 years for Tretayuga, 864,000 years for Dwaparuga, and 432,000 years for Kaliyaga beginning in 3102 BC. These periods seem interminably long.
Sri Yukteswar in his "Holy Science" believes the periods are exxagerated and are in fact much shorter(by a factor of 360), and that the last Golden Age actually lasted 4800 years(11,502 to 6702 BC), the last Silver age lasted 3600 years(6702 to 3102 BC), the last Copper Age lasted 2400 years(3102 to 702 BC), and the last Iron Age also lasted 2400 years(702 BC to 1698 AD).
He says we are now in the early stages of a new Bronze/Copper Age(1698 to 4098 AD), hence the slowly increasing knowledge and lifespans.
If Sri Yukteswar is correct, then we are well on our way towards achieving life spans of 1000 years old and body heights of 11 to 12 feet tall again, even though at the moment that does not appear to be the case. Hopefully, by the mid point of the Bronze or Copper Age(2898 AD) we will do so.
As we upswing through to a new Golden Age, humanity will once again be huge and long-lived and walk with(or as) the gods again, and this plane will appear smaller and easier to walk through and master.