Deb,
The templates that we use on the Auctiva lister page will automatically use 400x300 images. If the original image uploaded by the user is larger than 400x300, there will be a "click to supersize" link right below the 400x300 image that will open up a new window and display the full size image.
A user can still use the full size image, or any size image for that matter, in the auction description. In our description editor, there is a button (yellow box on bottom right) that will open up the Auctiva image selector. You can then insert any of the images you have uploaded to Auctiva and they will be in the same original size you uploaded them.
A user also has the option of using externally hosted images if they would like on the Auctiva lister page.
Hope this helps,
-Kevin