Your best bet is just take the boots in with you so you can make sure they fit.
For example, Rome bindings cover a wide range of boot sizes. Many in here will up size and go for a L/XL instead of the S/M. But I took my boots in and with some adjusting was able to fit my boots into a S/M 390 Boss.
Another thing to take into account, do you have BOA? If yes, you may need wider bindings. Also, check to make sure you can get to the knobs on the side of the boots; sometimes the strap gets in the way.
Thanks! I don't have boa, so i'll just stick to a regular bindings and nothing wider. The advice that very helpful; i'll bring in my boots then.