We're generally talking between 15 and 20 feet if left alone - scaled down for Johto and nowhere near the size of the Prism Eiffel Tower, but still pretty sizable! Now, if they've had energy blasted into them for whatever reason (ie, via special attacks) then that's going to affect their size, since they are reactive to that sort of thing and it just makes them grow faster and taller, as stated in the post.