I'm not sure what size your Bella is, but I can tell you how our black Alex bag (the large size) works for us. Zoë is about 5.5 pounds. I too was worried what size bag to get. In the end I measured
Zoë and picked the large, since I wanted her to fit with some extra room instead of it being too small. (The small was too small, height wise, for her so I gambled with the large. For us it's perfect (well something a tiny bit smaller would work too, but that doesn't exist!).
There's plenty of room for her, so I always throw in some extra toys. She doesn't play with them but if she ever wants to she can.
I'd measure Bella and take it from there! Because weights can be a good guideline but they're not exact. Good luck!