Some super-heroes can grow to be quite large, such as Giant-Man and Goliath, but which hero (size-changing included), is the biggest ever seen? No villains, please, I don't care about Galactus.
In the Marvel universe, you'd probably struggle to find a being larger than The Beyonder. In his home universe, he was the entire universe.
As an aside, within the DC universe we have Mogo the sentient planet. He's a member of the Green Lantern Corp. Not quite universe-sized, but still pretty chunky.