There are several sizes for this product - I ended up purchasing the 3" one a few years ago when I had a Roborovski hamster. Those guys are tiny little critters and that was the smallest I could go. The ball served its purpose in keeping him running but the locking mechanism wasn't also secure. I did go through probably three of these. Every once in a while, if he ran a certain way, it would come loose and you can imagine how hard it is to catch a 1.5" hamster that has escaped! It also collects a lot of dust so must be cleaned after every use. Maybe it's less troublesome for a less active hamster type?