I agree fully with Nasim. I print up to A1 size on a Canon ImageproGraf 2000 printer. A full resolution file at 300ppi will give a buyer maximum flexibility to print a variety of sizes. However, that may be giving away too much. I would offer two, three or more sizes (each with pixel dimensions to be able to print say, 6x4, 8x10, 11x8.5, 16x12 at 300ppi) at different price points. If the customer wants more pixels to make bigger prints, he/she should be prepared to pay more.