I would go by weight, 15% of your BP's body weight to be exact
(see post I made in your other thread)
The issue is that even here in the US while to progression of names is the same ( pinkies = smallest, then fuzzes, then pups) the size of rodents varies. So you might get 10 gram rodents called pinkies from one supplier but 5 gram rodents called pinkies from another...
By going by weight of the rodent in ratio to the weight of your snake you will have an accurate method of determining prey size.
* this only work for smaller BP's with adults a rodent that weighs 15% of its body weight would be too large.