Size L - good for one cat
It's a bit narrow, but my cats seem to like it. They wander in and sit in it sometimes, but that could be because they don't know what it is yet.
However, I'm uncertain about the mesh fabric. One of my cats will easily eat through it if she's in the mood.
Size L is about good enough for one regular cat. I wouldn't want to take the larger of my two cats in it.
Good cat carrier, though the large carrier is only for a medium cat (4-6 kg) and not suitable for a large cat (8-10 kg) in my opinion, and it's not because I feel that my cats should have a lot of space around them during travel (my large cat prefers to be crammed actually, but it would still be too small for him).
It's nice to carry over one's should, hangs comfortably.
I was disappointed to find out that the sides of the bag doesn't contain hard cardboard, and thus is easily collapses if one opens the big opening (not if one only use the small opening in the same end).
The pocket ind the end is practiacle for ones vaccinations papers and things like that