I don't have it in front of me so I can't give you exact. They are small enough for baby to easily pick up, but too big to be put in their mouths. Every baby tries to chew on the balls and there hasn't been a problem.
2 inches across. They're light weight, hollow, plastic balls. Not sure if it matters or not, but our son didn't use the actual balls all of the time. Everything and anything that would fit into the open area would go in there (stacking cups, sorting block, jumbo legos, etc). He just like watching things bounce around in the belly. But still, the toy was worth every penny!!!