There is no way to tell how big the small mice are. I agree though, try and feed just 1 appropriately sized rat or mouse. I would really work on getting her switched onto rats soon though, some B.c.i. have been known to have a hard time with the transition from mice to rats.
She looks great and looks to be just about right body weight from what I can see. Congrats with such a nice baby and keep up the good work!!