I got mine at Joann and had to try them on. They only come in med or lg (the size is noted at the bottom left of the package). My hand is small so I just tried on all the mediums and got the best fit. Fit did vary from glove to glove, so you really need to try it on before buying one.
I've had some nasty cuts in the past (not rotary) and don't want to risk a rotary accident. The glove makes me feel so much safer and confident. Bonus -- the nubbies help steady your ruler as you're holding it, so no slipping.
These are pricey, but Joann always has a coupon available.