If she can easily take large rats she can probably eat an extra small rabbit. There are rabbit pinks which are about the size of a large pup so after that they have a rabbit size for everyone.
If you have a scale you can weigh out the rat and then just purchase rabbits within that range. I'm not sure if you are ordering from an online feeder source but I have heard they will try to get as close as you need if you let them know.