Winamp can be forced into full screen, you just have to play with it some. This is only for version and 3 and 5 as the skins are scalable. It will take up the full screen but it isn't a "full screen " mode.
If you look at the link you'll see the in the fith picture how I was able to get winamp to take the full screen. This was done with Winamp 3. I've now moved onto winamp 5 which works even better.