I can get rabbits as small as kits, which are about the same size as a weanling rat. I start offering those to retics as soon as they are big enough to take them, usually alternating between a weanling rat and rabbit kit for each meal. It's rare but sometimes a retic will be hard to switch over to rabbits; I bought one adult female a few years ago that was knocking back 4-5 jumbo rats per weekly feeding and she wouldn't touch a rabbit. I did eventually get her converted by sewing a rat onto a rabbit's head using absorbable suture. Looked like heck but it worked!