I saw one seller who uses 640 x 480 (l x w) so I resized all my photos before uploading them to Auctiva. And then, I did a listing preview using my resized pix and preview showed horizontal scrolling.
So now, I am manually resizing all 1000 or so photos into maximum size 500 pixels. I already deleted original photos off my digicam. I just wish Auctiva adds a 'batch edit' function.