Q:

How tall would animals, trees and "humans" grow on a planet with those characteristics?

I am working on a planet were life (trees, plants, food, animals, humans) would be bigger/taller than Earth. I have learned that oxygen level or gravity, for example, could help change the size of life forms.
So here are the characteristics of the planet : 
Nitrogen : 72,4%, Oxygen : 26%, Argon : 1,05%, CO₂ : 0,5%
I have settled the air density at 1.9 kg/m3, so approximately 148 892 Pa and 1,4 ATM (but I have doubts on whether it's good or bad for creating taller life forms). And I still don't know how much I would have to lower gravity (0,9g or maybe 0,8g ?)
There is a big continent near the equator, so there is a lot of huge forests (one of the reasons the level of oxygen is at 26%). A big part of the forests are near mountain range and/or coasts, so there are a lot of rivers, and so very fertile soil. These forests have very rich and abundant biodiversity (with different species of plants and animals than Earth, that I still have to create). And the trees would have similar behaviors than Redwoods (https://www.latimes.com/archives/la-xpm-2002-sep-01-adme-redwoods1-story.html), having water sources from both the ground and the canopy.
So considering all those characteristics, how tall trees, animals and "humans" could grow ? (bigger it is, the better)
Thanks for all the answers you will be able to give me !

A:

The reduced gravity would help a bit to produce bigger trees but I wouldn't expect more than a 10-20% increase. What would make life very difficult would be the high level of oxygen in the atmosphere especially at the higher pressure. Wild fires would be an enormous risk as would storms so the giant redwood trees might not get a chance to grow to maximum height.
Whilst trees greatly benefit from being taller the same is not true of humans and animals. They might well remain at roughly the same size. The extra availability of oxygen and reduced gravity might allow them to be smaller as less muscle mass would be needed and smaller lungs. or the same sized muscle and lungs and a greater endurance and performance. Very hard to say as it would depend on the evolutionary pressures over millions of years.

