I have run in to issues as far as print quality when selling a print that was larger than the original, and have almost lost a sale, so now I too try not to sell prints larger than the original just to be safe. As Beth said, FAA will only sell image sizes that you have provided your mark-up price on. If you leave the price blank on larger sizes when uploading, those sizes will not be offered.
Welcome to FAA hope you like it here as much as I do!!