Yeah a lot of this is gonna hinge on how the animal likes to coil up. I'm not using RBI but here's mine in his current hides. Pretty sure he'll fit into a medium snugly and he was around 770g I think last weigh-in?
Like mine always likes to have his middle mass jutting out of the door like that and tends to turtle it along as he settles in, thus often spinning around his hide. Which leads to tantrums since then he can't see out the door.