It sounds like they're selling you smallmice, not adults. Check out this size chart from an online f/t seller.

If you can switch her to f/t you might be able to save money by ordering in bulk, it's what I do and I love the ease of picking out the size I need from the freezer. The only tricky part is figuring out how many of each size to order (I get about 3-4 months worth at a time).

If you can't switch to f/t, you might want to consider buying the mice in advance and "fattening them up" before feeding, but that could be more trouble than it's worth.