It would be best if you could get a scale, take the guesswork out of it. If she really is as big around as a fuzzy mouse, she's very skinny . BPs start on hopper mice or rat fuzzies and should be eating adult mice or rat pups by the time they go to new homes. So a mouse fuzzy is normally too small even for a new hatchling.

You can get a digital postal or kitchen scale for around $20. Weigh her, than find a feeder that's about 10%-15% of her weight.

You should feed her in her enclosure. BPs get stressed from being moved to eat. Plus, they can digest bones, so a little aspen isn't gonna hurt . Sand and rocks are what cause impaction/blockage, not a little bit of wood shavings. But if you're really worried, you can put a paper towel down, or completely ditch the aspen and just use paper towels.

Yeah, if you've been moving her to feed her, that could be one reason why she hasn't eaten. It rarely works with BPs.