If higher levels of atmospheric oxygen can lead to larger animals, can it also lead to larger plants?
Oxygen is good for animals because our basic metabolism is this:


  High energy carbon molecules + O2 → energy + H2O + CO2


Plants do that too at night, but during the day, they mostly do this:


  High energy photons + H2O + CO2 → High energy carbon molecules + O2


Rubisco, one of the most important enzymes in photosynthesis, can bind to O2, leading to less efficient photorespiration, instead of photosyntheis.  Rubisco is often a bottleneck in photosynthesis, in large part for that reason.  So high oxygen isn't helpful to plants.  In fact, C4 plants and CAM plants have evolved more complicated physiology to minimize the effect of O2.