Usually, the portraits will be 9 inches by 12 inches, scanned at 300 dpi. My scanner can't handle much bigger, though I hope to invest in a bigger scanner.
The more you donate, the bigger the portrait. After $500, you can say whatever size you want to work at, we'll come up with a more interesting and elaborate pose, and we can have multiple sessions. Again, the more you give, the bigger the painting.