A normal adult mouse is about the size of a rat pup/crawler. Typically around 3 inches long and about 25 grams. Sometimes very large retired breeder mice can get the size of weaned rats, closer to 4 inces and 35 - 40 grams.

A great site for comparing sizes is www.rodentpro.com. If you "browse products" you can view pictures of each size of rodent as well as see weights and lengths.

Hope this helps.

-adam