Joseph Wisniewski , Dec 19, 2009; 09:22 p.m.
You can't. You might as well drop the idea of selling files at all those different sizes right now.
You don't say what field you're working in, but with your very small print sizes (4x6, 5x7, 8x10) and your general lack of experience, I'm guessing something like youth sports. Now, a photographer is concerned with the file size and DPI when he prints, or sends a file off for printing, but the typical "soccer mom" isn't. They will take your "web size" file and print an 8x10 for the fridge. The jaggies and pixels won't matter. If they notice them, they won't blame themselves for buying too small a file: they will blame you for your poor photography.