Rex,
If you have had your bp for any amount of time, and it is eating rats, you should be able to tell a rat from a mouse. Mice are TINY little things, even what they call a jumbo mouse is easy to tell the difference from a rat. A mouse looks more fragile than a rat- they are much smaller, has a thinner tail, its facial features are tiny. Rats have thicker tails, their noses are longer... what they sold you was either a big mouse, or a very young rat... can you post a pic, it is very hard to answer your question without one seeing as though a description can be hard to picture when it comes to rodents.