According to the Alan Dean Forser's TFA novelization (Chapter X), he was tall.
Seated on the raised platform that was the focus of the chamber was the blue-tinted holo of Supreme Leader Snoke. Tall and gaunt, he was humanoid but not human.
Seated in shadow, the tall, slender form loomed over the other two men.
Lest this be taken as "Hologram effect only", WoG is that he is tall as well:
But Neal Scanlan, chief of creature and droid effects, told PEOPLE, “This character is much better executed as a CGI character. That’s just a practical reality when he’s 7-foot-something tall; he’s very, very thin.” (source)