Ok, I realize this is a little different then what you originally asked, but I thought that since you're thinking of raising rats/mice I'd give you a size comparison of a few adults.
My rodent breeding is pretty modest, and at this point I only have one male mouse and one male rat.
Here are some comparison pictures of my male mouse and Robin, my male breeder rat.
Sorry that they aren't the exact same shot, that was my original plan. It was really hard to keep Robin in his designated area!
Male rats get HUGE, Robin was born in December, he's half a year old. I'll spare you a "poop comparison" picture between Robin and my male mouse.. hehe
Honestly though I have my rat and mouse colony in my room and they really don't smell too bad. I use cat litter at the bottom of my mouse's aquarium and in the pull out shelf at the bottom of the rat's cage. It's a life saver.
Here's some closeup shots of the two, but it's not as great of a comparison because one picture is vertical and the other is horizontal.
If you're consitering getting rid of the mice all together and replacing them with rats think about how large your cage is. A spacious home for 3 mice gets pretty cramped for 3 rats. Robin grew up pretty fast, here's one of the first pictures I took of him. I can't believe he used to be that small!
I hope that that wasn't too much rambling and at least a little bit of help! Good luck