You should get the size of horse you're most comfortable with.
I'm only 5'3 and petite, but I like big horses better than I like smaller ones. I feel safer. I can ride 13hh ponies, but I prefer horses 16hh+ and the bigger the better. I look like a pimple on a pumpkin on the really big ones but I'm comfortable and that's what matters. I know tall people who ride small horses, and provided the horse is able to take their weight, it's no problem at all.
The only reason I can think of for someone to really NEED a horse that takes up their whole leg is if they jump high. Hitting your foot on a jump hurts. A lot. [I used to ride a 12.1hh Welsh, it happened to me regularly]