The large size is perfect for Syrian Hamsters
I have had the small size one in the past and it is tiny, I would only recommend that for mice.
The large size is perfect for Syrians, rats, gerbils, degus, etc but on the small side for guinea pigs.
My Syrian loves his, he uses the bottom for sleeping in and the top as a look out & play area.
It comfortably fits with space to spare in an Alaska cage but is a bit tight height wise in a hamster heaven. I will definitely buy them again in the future