In theory objects used as entities can be any size you want.
Increasing the size of entities should not increase poly counts.
Be aware though that increasing sizes of enties will increase the size of polys and the holes between the poly vertex's which may affect collision of those objects. Extremely large entities close to player can sometimes have camera clip probs and and extremely dymamic entities could have - well orther behaviours I have not encountered.
As there is no imposed limit I am aware of - go ahead and try it.
