His official height from inception was over 7", so for the way Kirby drew him it was fairly accurate. The problem is that most artists have taken liberties with that. Mike Deodato Jr. is my favorite Hulk artist bar none, but I'd never try to convince anybody that he draws the Hulk at anything below 9" tall. Want an example? Just look at this pic Louis Leterrier has on his MySpace:
Amazing Hulk photo, but if you think he's 7" tall in it you should probably work on your sense of scale.
On a parting note, here's a fun fact: While many think that a 15" tall Hulk would be unable to move in our world, neither would a 9" tall Hulk. The most plausible height for Hulk would be Kirby's rendition, around 7". Just a little something to think about when academics try to tear into comic books about inaccuracy.