So get a food scale and a large bowl and weigh your critter..

We brought home a new juvenile from the pet store about 2 months ago.. she weighed 60 grams and she was able to take a pinky rat (8-13 grams live weight by my scale).. Previously she'd been on 5 gram live hopper mice. As of yesterday she is 100 grams and just graduated to the smallest fuzzy rat in the feeder bin.. about 20 grams.. might have been a little oversize by weight for her, but she took it like a champ (and girth wise was appropriately sized).

Im not posting as an expert, just sharing whats working with our juvenile.