1) They are usually sold in UK sizes. UK 10=US 11, make sure you know which they are marked in when you order!
2) Order up at least half a size. If you are planning on wearing two pairs of socks or liner/VBL/sock combo, definitely go with a full size. If you wear a thin liner and a regular sock, or just mountaineering socks, and don't have particularly chunky feet, half size is plenty. They are high volume.
3) They shells are sized in whole sizes only. A US 11.5 will have the same shell as a US 11, but with thinner liners.
4) Don't sweat it too much. They're big chunky things and half a size isn't make or break.
Here's a great summary on sizing:
http://whittakermountaineering.blogspot ... boots.html