In reply to Roberttaylor:
In my limited experience (a handful of trips up to about 6000m) mine don't swell much, particularly when active. Certainly much less than the difference between boot sizes, and the more significant factor is the type of socks I'm wearing.
There's a lot of variation between individuals though (consider for example how much some people's feet swell up on an aeroplane) so my experience may not be the same as yours and I've read in the past on here about some people who take the half size-up from their boot size at sea level.