I got the small size, as it was listed as up to 25 pounds. I have cats that are 7 pounds and 15 pounds and neither one fit into it, even the smaller cat. If I stuffed one of them in (and have you ever had to stuff a cat into anything?) I might be able to get them on a short a to b trip, but for anything longer than 15 minutes, they would be going nuts, as there is no room to move around comfortably. Use the measurements, not the weight, to determine whether your pet will fit.