I agree with Mr Kewl And, it will be so much easier to feed an adult bp rats. Since, i'm pretty sure no mouse in the world is going to weigh the same as a small rat. like a 70 gram'r. Unless of course you have access to some jumbo mice. Also, make sure it is a big healthy mouse. A fat mouse, will not provide as much nutrition as a healthy rat the same weight. It's easier to switch a young bp than an older one from mice to rats.

Also, when he reaches a certian weight his mice quantity will increase. It's more cost efficient, for me, to buy a 70 gram rat than 2 big mice (or 3 smaller ones).