At Congress, my 15.2hh paint was completely dwarfed and placed last. He has very "western pleasure movement", so he has a shorter stride than most of the horses out there.
The baby I have now I plan to use in all-around, but her sire is one of the big HUS stallions out there. She should mature to 16hh, which is a good size for an all-around horse. Still small enough to do the western, but still large enough to do the english. If you were just going to specialize in english pleasure, I wouldn't look for anything smaller than 16hh, and even 16hh might look small in the upper levels.
Here is a video of Lily's stallion performing. You can see that even though he is 16.1/16.2, some the horses that pass by him make him look tiny!: