aje, this may be way more info than you are looking for, but it is neccessary in explaining what size foot I truly have. Unweighted and in the neutral position I am a 7.5, weighted I am just barely a size 8, I have a medium-high arch and practially no pronation, but my heel to ball measurement is a size 10. Based off of those specs I am a 8.5 in most skate shoes, 9 in some running shoes, and I can't fit in a Nike shoe.
The more a foot pronates, the more it elongates when weighted. This is why basing your boot size off of a shoe size is a pretty big mistake...because every foot is different. There are people that wear size 12 shoes but ride in 10 boots. Shoes are made for walking and we typically don't want anything touching around the toes/forefoot so most people wear shoes about 1 full size larger than their actual foot size (shoes are designed with by the heel to ball measurments and people measure their foot by total length). Snowboard and ski boots are made for connecting your body to your equipment so a secure fit is imperative. When you put a boot your toes should touch the front, lace the boot, flex it, and with the knee bend and boot flexed your toes should release from the front of the boot (no longer touching). If a foot pronates a lot, this feeling may not be relieved...that is why people feel a boot is too small. That is why footbeds are so important to a proper fitting boot. A supportive footbed will help minimize pronation and maitiain foot shape so that you do not have to size up (and get in teh wrong size boot).
Too much info, right. Short version: get your foot evaluated and fit by a professional and try a lot of boots on so that you know exactly what size boot/foot you are.