How do I know what size scan I need?
You can calculate the size of the file you need using this formula:

( (width x ppi) x (height x ppi) x 3) ÷ 1,000,000 = File Size in mb

You multiply the width and height (in inches) each by the resolution you
need in pixels per inch, multiply those two totals, then multiply that total
by 3 for the red, green, and blue color channels. Finally, divide that total
by one million to get the number of megabytes.

So if you want to make an 8x10 print and your printer needs a file at 300
ppi, it would look like this:
( (8 x 300) x (10 x 300) x 3) ÷ 1,000,000;
2400 x 3000 = 7,200,000
7,200,000 x 3 = 21,600,000
21,600,000 ÷ 1,000,000 = 21.6 mb