When adding products with images you can set a minimum image resolution on a per product basis - in the Advanced Section when adding an image
We produce artwork at a minimum of 300 DPI but store image resolution in pixels to accomdate dynamic resolution checking
eg If you have a large image that is only say 75 DPI but you put on a product that is 5 times smaller than your image size the image will be good enough quality
To calculate your the pixel dimensions for your product please use the following calculation
Image Area Dimensions x 5 Pixels per mm
These are based on 5 pixels/mm for sublimation and transfer products and 6 pixels/mm for Photo Print product.
Here is a spreadsheet with some examples of typical products