Why would you want to style it? It's intended to be read in a news aggregator, which will handle the presentation of the feed itself. Tinkering with the feed output might invalidate it, meaning that aggregators fail to read it properly. It's not supposed to look sexy, it's supposed to be syndicated.

That said, you can style pure XML with CSS, though that's not recormended.

I would like to have a third image size just for the feed. Not the thumbnail size or the original size.