I don't see anyway to do what you want. Where would the bigger art go? There's no vertical room to expand it. You have to make the cell where art's displayed bigger so it can be resized.
Is that possible?
I don't know, I don't use Winamp. I can do that in Foobar2000. All fields are fully customizable. I /think/ Winamp can be customized, but it's been years since I used it, and I didn't work with it much. Once I found Foobar, I never looked elsewhere for players :^)
I do like foobar. I just don't feel like messing around and customizing it forever until I get it the way I want it.