Thanks for all your replies. Looking at Carboniferous Rainforest Collapse, it says that with climate getting colder and drier, the tropical rainforests were devastated, and this caused the oxygen amounts to lower and carbondioxide to rise. Still, there doesn't seem to be anything told about the sizes of plants, and I'm guessing that only biomass changed, not individual sizes.
We can't ignore the CO2. If you check this chart(on page 9), comparing those values with current ones shows that during Carboniferous Period the CO2 levels were 2 or 3 thousand times higher than current levels.