My dwarf hamster is 3 weeks old. When will she be fully grown?

I just adopted a campbell's dwarf hamster from a local breeder. She's currently three weeks old and TINY! So small compared to dwarf hamsters that I've seen at the pet store, and I'm starting to worry that she's a robo rather than a dwarf hamster. Since she's only three weeks old, does any one know roughly when she'll be fully grown? And how much more do you think she'll grow? ie Is she going to double in size?