The biggest horse that I really rode was my friend's 6 year old, 17.2 hh thoroughbred gelding. I rode him in a lesson when my horse was off.
The biggest horse I've ever been on was 18.2 hh Clydesdale. I'm only 5'1" with short legs and I have to say, I felt like my legs where going to fall off after I got off. That thing was WIDE!