It should be about the size of an x-l mouse but different retailers can label things differently. There's a lot of discussion on the merits of rat vs mouse on the forum but I think the general consensus (if there is one!) is that it is better to feed one appropriate size rat than multiple mice, and even some evidence that they will grow faster too. Not all BPs will take a rat if they are started on mice although I think most will if people are prepared to use all the tricks. If Ruth lives on mice for the rest of her life she will be just fine too.