Tallest I've been near was an ENORMOUS warmblood called Fleeting. He was huge. I commented that he had to be at least 18hh and one of the girls working for his owner said he was 18.2 or 18.3... I couldn't reach to get the saddle on him the day I tacked him up! Riser was all wonky, they ride all their horses with some form of riser. Fleeting wore a flat one.
I am 5'2-ish and one of the girls working there came up to my armpit... I commented once to one of the other girls that I would LOVE to see Emma ride him XD she would look like a pimple on a pumpkin!
Tallest horse I've ridden was a 16.3hh TB mare who I ride now and then for her owner. My girl is a big girl, she'll be 24 months old in 2 days and her rump is already 16hh+, she's around 15.3 at the wither... and she has a LOT of growing to do.