I would be inclined to sell the same sized shirts as a lot, if only because it drastically reduces the time you have to invest to list them. If you sell them separately, you may realize a greater profit in the long run, but you will most likely sit on the less desirable color combinations for a lot longer. When listed as a lot, if a potential buyer likes five of the shirts but isn't crazy about two of them, you are still likely to make the sale. Just be sure to measure each of the shirts ahead of time even though they are all marked the same size. I have had experiences where shirts by the same company shrink at different rates depending on how they were cared for and where they were manufactured. Good luck!