I did a "/summon Giant", and then tried to name him with a nametag. Unless the name is very long (or the Giant turns his head just so), you cannot see the name floating above his head, because in fact the nametag hovers inside the Giant's head hitbox (and portions of the name text sometimes clip outside his head). The name should appear above his head instead.

Aside: a renamed Ghast also has the name appear perilously close to the top of its head.

(Honestly, this is a minor bug, I was just hoping possibly the text display of the name might also wind up scaled up to giant size. I was trying to come up with clever ways to display hovering text (a la renamed items in item frames) on a large scale (e.g. was hoping to hide a renamed Giant in the floor, so mysterious large floating text might appear).)