Best bet is honestly to try them on in a store.
Personally, I would rather buy things like footwear in a store as opposed to online because different manufacturers have different sizing schemes. This is actually problematic for someone like me because I sometimes need wide shoes and sometimes regular shoes (I'm kind of in between wide and regular). For example one company, lets say saucony I would need 9.5W but another like Nike or Asics I would only need 9.5 Regular and the shoes would stretch out after a few times of wearing them.
I know this is true for shoes but I don't know if it carries over to boots however if they're new they should fit a bit snug around the foot (only SLIGHTLY SNUG, not like vice grips around your feet) and stretch out after a few times of wearing them till they're a comfortable fit.
EDIT: Zeon beat me to it, but I was also going to mention that you should figure out which sizing scale they're using, either NATO (mondo point I believe), EU, UK, USA, etc.
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READY TO >> RACE