Thanee said:Is Polymorph a magical effect?
yes, it is a magical effect.
Thanee said:Is your size increased after you Polymorph from a medium size to a large size?
No, you have the form of the new creature, your size does not change. You become a new creature and its size may or may not be different than the size you were before, but it is immaterial.
There have been a lot of threads on this already, but the point is pretty clear. Enlarge person changes your size, polymorph does not.