You think a medium to large rat is a small meal?? What would she normally get? A rabbit?!

Go for a just-weaned rat. ONE of them. See how she does and in a week, give her another one.

She will grow and maintain her weight very easily on either 3 adult mice a week or a small/small rat. They definitely don't need large rats to maintain. That will just cause her to regurgitate again or just go off feed for a few months.