The appropriate size will not be a number it will be an overall shape, as in plump not triangular shape which will be achieve with every 5 to 7 day feeding, feeding the appropriate size prey (= the girth size), at 55 grams we are talking about hopper mice (2 weeks old eyes open and hopping around)

Don't feed too often or too much in animals that have been neglected, starved, severely underfed it can be dangerous (not saying that this is what happened here) it could simply be an animal that just hatched.