Reed Richards I believe can stretch up to 125 feet, or somewhere around there, taller than Goliath and Giant Man/ Yellowjacket.
Galactus. Or at least he'll always be a superhero to me. He's eating the bad planets that would ultimately destroy the whole universe if they were to be left unconsumed. Earth was supposed to be just an appetizer. Silly Fantastic Four. How rude.
The height Reed Richards can stretch up to is unlimited, but it's not his natural height, so he's not the tallest. Hank Pym has to use a belt to get to any height above 6 ft. (his normal height). I know for a fact that Iron Man is 6 ft. 1 in., so Hank isn't the tallest either.
In terms of natural height, Cable is one of the tallest at 6 ft. 8 in.