Good information on room sizes is very difficult to find.
The Unofficial Guide says GF standard rooms are 400 sq ft.
Although the Unofficial Guide has a lot of good info in it, I am skeptical of its room size information as it varies from other sources in several ways, e.g. it claims that Caribbean Beach rooms are the same size as those at Port Orleans, even though any number of people and sources recognize that Caribbean Beach rooms are larger.
This site (wdwinfo.com) says 440 sq ft, which is what some other sources say. Perhaps the only way to be sure is to stay there and bring a tape measure!
Info on the Polynesian is more consistent, usually 409 sq ft (Tikiman says 402). And most sources agree that Contemporary rooms are about the same size as GF, with 436 sq ft commonly given.