I would check alldogboot's site:
Dog Boots | Dog Shoes & Dog Booties
There is a printable chart, for sizing, and if you have any questions I would shoot her a PM she was happy to help me size Kaos.
It's getting so close to winter and we haven't been able to order our winter boots yet, but we will be ordering ours from there. I hope the snow holds off for 2 more paychecks. lol