As a general rule: the bigger the better! Use your common sense. Think about how big you are, and how big your house is. Now imagine you’re the size of a mouse, and think about how much space you’d like to live in. My single boys get a cage with floorspace about 45cm by 30cm. I personally wouldn’t keep any mouse in less space than that – though I might keep up to 3 in that size. I don’t think 1 mouse needs less space than 3 mice – so if you think your cage is too small to comfortably house 3 mice, then it’s probably too small. If you’re stuck for space, then go vertical! Shelves, ropes, hammocks, hanging toys, all add to a mouse’s environment, without necessarily taking up more room.