You always want to watch careful. Have a hose or bucket of water ready, that's helpful if a fight happens.
The dog should be UTD on vaccines and well socialized as well-ideals.
Also make sure you both understand dog body language and proper dog play. It won't be helpful if your screaming and breaking up normal dog play. And it wont' be good to miss signs of aggression. So if you aren't sure, let us know we can point you to some resources on dog behavior and play.
But I'd try it for sure. Size doesn't matter. My bf's mom takes her chi's to the dog park. They are particularly in love with a big golden retriever-who is more then 10x their weight. Mikey is 40lbs and plays with 2lb'ers and 200lb's -size doesn't matter UNLESS they fight-which you want to prevent