Do you mean zoom in the ebay gallery or zoom on images in the listing template?
For the zoom on the ebay gallery, i have never managed to make it display larger. I just assumed i was doing something wrong.
For listing images i don't use the zoom. I link directly to the full size image on auctiva hosting.
Here is one of my listings if you want to see how i do them: 3 hoop earrings
I'm not sure if its ok to link to my auctions, but if its not i guess a moderator will delete it. I'm not adding it for increased exposure. It's just there as an example.