If you switch to the "Big Bento" skin instead of the default "Bento", it will be slightly larger. If that's not enough (and it's not much of a difference), AFAIK you will have to make your own, modified skin. Winamp's "SUI" skins do not allow you to easily change the size of any part of the interface (one of many reasons they suck).
If you alter the Bento skin to simply make that section taller, I expect the album art will expand to fit. But that will mean manually modifying the skin files, and probably making other parts of the interface look terrible. There are plugins on Winamp's site that will display album art using the "video" box, but they do not work well at all.
Quite honestly I attempted to do something similar and ended up just switching to Foobar2000. MediaMonkey also displays album art at a larger (and more easily changed) size.