I purchased the smallest size (to fit in my car) for an upcoming 2 day drive. I have 3 cats and will rotate them from their carriers to this. It has room enough for one large cat, a small litter box, and a foodwater bowl. It will contain a (20 lb. or less) animal safely with plenty of ventilation. Super easy to pop open and fold back up! This is EXACTLY what I was looking for! I set it up in the bedroom for them to get used to, but before I could even throw some toys in it, they all wanted to get inside! So currently its serving as a new play hideout for them as well. I wouldnt consider leaving a cat outdoors in this without close supervision though. Although made well, it is VERY lightweight.