I agree, with the above, as a dressage rider myself, and also on the smaller end, there are plenty of smaller horses doing well in dressage. I would focus more on ability, training the horse has had, how it moves, if it's done any shows, and how it's done, and less on finding that perfect height for dressage. There is no such thing. That being said, while I love my friend's pony, she's a large welsh pony, and I didn't mind my client's 15.2/3 hand WB gelding, I personally feel much more at home on 16-17 hand horses. I can get them to listen effectively, and while I certainly don't look "big" on them, I also don't look too horribly small. But after my friend getting her pony, I've realized that I can also enjoy riding the ponies, and a lot of adults are just too big so they have to stick with horses. Ponies are interesting animals, and I do have a blast riding them. I wouldn't let height determine what you look at. Find what suits you, is comfortable to ride, and can do what you want.