In reply to dgim: ....this sounds about right. But there are some things not picked up in the thread, by other comments. The criteria for boots pull in different directions.
1 pain: boots too tight (you can get them stretched)
2 blisters: probably too loose (maybe new insoles or heel grippers) - note that with plastics and some B3 boots you can put in a sheet of neoprene to reduce the volume - this can work a treat with double boots - I've known someone cut up a karrimat as an emergency measure and then kept this in since it worked so well.
3 not freezing your feet (stretch, but can't do much about the volume over the bridge of the foot, and if too tight here you can cut off the circulation to your foot, since big veins go over this area - and this is really bad news - if this is a problem, it's probably new boots).
4 yes, getting cold is a disaster and can risk your toes to frostbite, but no-one has mentioned the issue of edging and not falling on snow plods. So many people ramble on about arresting, but the thing is not to fall in the first place - this is SO important. Now, I've had boots where the front lug seems to stick out a mile (scarpa mainly) and I just find this decreases the precision of footwork - which is a major issue.
5 stability - I have merrells which have a very wide and soft sole unit - which is brilliant. And some asolo b2s which are good volume in the shoes itself but very narrow in the foot sole unit, which is good for edging but not so good for snow slopes. The worst I have is a pair of raichle mount envy's, which have a weirdly unstable but very stiff sole unit, very bad for stability when walking and jumping, hopping from rock to rock. They are going.
These criteria really do pull in different directions - tight for precision, loose but not too loose for insulation, etc etc.
So...I have one foot at 41 and one at 42 - which is a complete pain - and have some pretty close fitting garmont B3s for precise work (they have been professionally stretched) and some 42 nepal extremes, which fit brilliantly with one pair of thickish woollen alpine socks and the standard insoles. They do not restrict bloodflow oevr the bridge - which is vital for keeping your toes.
Sportiva seem to be the same as your normal size (but yes the evos are a very different volume and fit - narrow in the heel and tight over the bridge); scarpas seem to be huge (though not the mt blancs) and asolos are supposed to be narrow but aren't necessarily (my double AFS101s at 42 are huge)...
try them on for hours in the shop ... and you'll still get it wrong sometimes...which hurts the wallet a lot...