Without seeing them, I can only guess that you'll be fine for 5x7's. However, download a couple and give them a critical once-over @ 300 ppi (pixels per inch) and then a 150 ppi. Ideally, you want to be around 300 ppi when you print (250 is fine) but I don't suggest you go below 150.
When you select the pixels per inch (in a photo editor), your photo should resize automatically and that would be the size you're going to consider printing at.