If you want to take one image and put it on a lot of different products, we recommend the following:
10" x 10" @ 200 DPI
(2000 pixels X 2000 pixels)
While it won't fit on all our products, it does fit on all apparel items, mugs and print products. You'll need multiple versions of your image if you want to create all our products, since products such as bumper stickers, hats, and clocks have different shapes and sizes.