My ds is in a sleeping bag Clyon & we have it just the right size like DueinAugust said. He seems to like it.
Also i know tummy sleeping is scary but just go with it. We were forever rolling ds onto his back & then at sleep school he decided he wanted to sleep on his tummy. He went from doing 30 min catnaps to 2 hour sleeps once we just went with the tummy sleeping thing. We still put him down on his back but just let him roll around & find his own position.