They get resized on upload to maximum (either dimension) of 768pix V x 1024pix H for their largest, the Supersized original. Note the 3:4 aspect ratio, which I find common to most digit camera captures.
Tip....I find that uncropped images are best done with my Kodak EasyShare set at 1.7MP (lowest setting on my 6.1MP capable cam). They get the fastest upload with the least amount of resize processing. I set my cam to higher for cropping on closeup detail shots, i.e. not to lose the image sharpness due to pixilation. I do almost all editing at my workstation to avoid degrading the image after upload.
Edit: I was looking for the minimum system requirements at eBay, i.e. was there a minimum monitor resolution recommended. I didn't find one, other than live auctions. However, I do recall an 800 x 600 recommendation with 1024 x 768 as best viewing. If I were working to worst case, I'd size the picture accordingly to display for worst case, the 800 x 600. I'd do that by setting my monitor to that setting and working with the picture and Preview until I got something acceptable with the template, borders, etc. I'd try a horizontal dimension of 650 pixels using the Auctiva imsge properties on the inserted pic, and see what that does. Note, some templates have very wide left and right margin borders.