If you have the old box your contacts it'll have all the proper sizes written on the side. Then you can get the proper fitting ones. If you don't have them, just call the last place you got them from. They'll have all that info on file.
I just go to my optometrist every year and ask for sample packs of a bunch of different kinds and use them for free. They get them free and don't mind, although I amsure they expect me to buy some sooner or later, but oh well... they should say something.