I think their pants MIGHT run a little small if I remember correctly... If shirts, order the size you would usually get. But the best option would to actually go to the store and try on their clothes :)
Their clothes are mass produced in sweat shops so no two are ever the same. If you're getting their jeans that are like $7-10, the sizing is terrible--I remember the last time I went there and tried on 3 of the same pants that were also the same size, and all of them fit differently. But from what I can remember, they run small. However, if you're buying their jeans that are in the $15-30 range, they are usually made with better quality and 'truer' to its size. So I'd say to just stick to your normal size if you're buying in the $15-30 price range. You can always return online orders in-store for store credit if you have a forever 21 near you.
