These are the sizes I usually refer to.
Pinkies 1-4 days old Not forming a coat yet. "Pink" in color at this age.
Fuzzies 10-13 days old May have started to develop a slight coat of peach fuzz.Eyes are not yet open.
Pups 14 to 18 days of age. Eyes still not open.
Weaned 21 to 25 days of age. These rats are eating and drinking on their own. They have reached the stage when it is time to be "weaned" from their mother.
Small 26 to 32 days of age.
Medium 33 to 42 days of age.
Large 43 to 60 days of age. These rats have reached sexual maturity.
X Large These rats have reached their full adult size.
XX Large These rats have also reached their full adult size but are retired breeders therefore some may have a higher body fat content than our extra large rats.
XXX Large As big as they get! These rats have also reached their full adult size but are retired breeders therefore some may have a higher body fat content than both our extra large and double extra large rats.

Source: http://www.rodentpro.com/catalog.asp...el=frozen_rats