It is my understanding that mice develop in the following stages:
Pinkies -> Fuzzies -> Weanlings -> Hoppers -> Adults
My guess would be that Hoppers may be considered as small mice while juvenile adults would be med and aged/pregnant ones would be large.
Brbs can handle more than you would expect them too and more so, these little guys transfer food into amaizing growth rates. I have a 3month old unrelated pair that handle an adult mouse a week.
Just experiment a little, a 2 week old BRB can eat a pinkie every two days. I'd take a look at the 'small' mice and get one of the smallest ones there are. Feed it and watch the belly lump. If it goes in the first day, too small. If you can still see it the fourth or fifth day after feeding, too big!
Good luck to ya!
P.S. - Check your classifieds or your state herp societies to see if there are mice elsewhere. You'd be surprised the number of people who breed 'em.
-----
-Iman
1.1 Sugar Gliders (Gizmo and Nema)
1.0 Ball Pythons (Spot)
0.1 Guyanan BCC (Ruby)
0.1 Brazilian Rainbow (Isaboa)
Loving to Learn
Learning to Help
Helping to Love
Stimulate debates, stifle arguments.
Please be nice always.