I have small feet, so not a great comparison, but my SG10's are size 7, although I normally wear a size 5.5-6.0 gym shoe. I first ordered a 6 boot, and they were a little too snug in the toe, so I exchanged them for the 7. The 7 is a little roomy, but ultra comfy. My hubby wears a 14 in gym shoes, and the 14 SG10 fit him well.
So I would say get a 12 if that's what you usually wear, or maybe a 13 if you have some 12's that are too tight. I wouldn't go down a size though.