Biosphere conditions for larger lifeforms.?

During the Carboniferous Period of prehistoric times there was a higher atmosphere oxygen content, and plants and insects did grow to fairly huge proportions: spiders grew to the size of a person's head, and centipedes grew ten feet long. Dragon flies had a three foot wing span. All of this was probably due to the growth of plant life during that period. The following period, the Permian was much dryer, less plant growth, and the insects were not so large. However, reptiles and amphibians began changing, some becoming more mammal-like. This was still sixty million years before the dinosaurs. You should be able to find information on this on Wikipedia.