We brought over a Maltese and a Lhasa Apso and both ended up in a medium size crate for flight. I found that different manufacturers gave different measurements to each size crate (xsm, sm, med, lg, xlg), so I hunted until I found the right size without to much extra space, but both had plenty plenty of room. Both could have fit in the small sizes but would have been bit tight and I wanted to play it safe and let them have some room in the crates.
I was told by BA that there has to be at least 2 inches above the head when standing up w/ head up. (I think they said 2 inches above), and I was also told they had to be able to turn around in the crate.
As a note to anyone: No collars are allowed on them in the crates while flying, but you can put the collars and leads in the plastic package that comes with the 'live animals package" you have to also get. Buy some plastic ties also b/c we were told and did have to use extra ties to tie down the top of the crate to the bottom.
Also..... if you put any kind of bed in the crate for dog, make sure that if he wiggles around and it could get on top of him/her that there is plenty room for the dog to be able to move it himself/herself back into place underneath. (can cause serious problems if bed gets on top of dog and remains there)
The people at the Animal Reception Center in London were all very nice and very good to all the animals we saw the day we picked our little ones up.