The Wii uses DVDs so games can be 8.5GB max if a dual-layer disc is used. But only a handful of games actually use one, so a regular disc is 4.7GB. Most games don't even max that out, I'd guess they average half that space. Some games, such as Animal Crossing City Folk and Super Paper Mario, use half of one GB.
The XBox 360 uses DVDs as well so its limit is also 8.5GB. Blu-ray has a limit of 50GB for dual-layer though I don't know if any use it.