The largest horse in recorded history was a Shire called Mammoth, born in 1848 in Bedforshire. He was originally named Sampson, but his name was changed when his owners realized just how huge he was. He was four inches taller than Big Jake, making him over seven feet tall.
Fortunately, most horses don't come that big (unless you're in a fantasy world and want them to, of course.
(This Belgian is rather smaller than Big Jake...)