I think the printers at FAA run at about 150dpi. As Floyd said, just upload the largest image you can (without digitally enlarging it if possible). I have noticed the sizes skip a few numbers here and there, for example you can't order a 22 (on the short side) print and some other increments are left out.
Because we set our own margins for specific sizes, I think the listing tries to limit the list while covering a wide range of sizes and configurations.