The general rule of thumb is to triple the 8 week weight...and more accurately double the 12.
If this is not a new breeding between the parents...your breeder can probably give you a better guess-estimate of her adult weight.
Personally I know several people with 4 pound range, I don't consider them "teacups".
__________________ Deb, Reese, Reggie, Frazier, Libby, Sidney, & Bodie |