Right now I have an 8 inch flying saucer wheel for my relatively small male Syrian, but I am planning on upgrading to an 11 inch wodent wheel as soon as I get the money for it. It's not super big but also a good size for Syrians, as they do evidently grow as they get older. The wodent wheel is a little expensive but I know that it's available in lots of places, idk where you live but you can order it online
hope this helps