I have been selling here for a few years and I have never NOT got a size flag.

My pics are about 1200 x 800. They upload fine and are easy to look at.

My scanner defaults to about 1350 x 900 for a standard paperback, so I would have to blow them up (with the consequent loss of resolution) to comply with the suggested size. I instead reduce them by 10% to aid in upload time, which doesn't seem to affect clarity.