Yeah galactus can change his size using the power cosmic. His level of power can be a factor too though, meaning he may be smaller if he hasn't eaten a planet in a long time. But he can change his size if he feels it necessary. Can be as tall as a sky scraper or as big as a star with planets orbiting it.
His original form is a dude named Galan before he got the power cosmic and he looked just like a normal human. I'm not positive if he ever reverts back to that form to interact with people though.
