I'm curious, how's the rules about keepin mice at captivity where you live? In Sweden we have animal protector laws.
The rules of keeping mice in Sweden says that a mouse in captivity needs at least 0.09m², if there more than one mouse in the cage they must have at least 0.018m² per mouse. Mice are packanimals and shall not be held alone if it's not really necessary.
The cage must be longer than 0.2m at the shortest side and higher than 0.2m.
That's the measurements you must follow, you can always have bigger than that!
There's also demands in how the inside of the cage must be, you shall have something on the bottom, most common is sawdust but there's a lot of different substrate you can use, as long as the whole bottom is covered, it's important so your mouse can be kept dry and warm and also have something to dig around and bed with.
Some kind of straw or hay to bed with is also required.
You must also have some kind of shelter in the cage. Somewhere your friend can hide and have some peace.
The animals shall have acess to fresh water and food all the time.
To keep the teeth in good condition you must have something in the cage to chew at, for example you can have wooden sticks.
The mice need an stimulating environment with possibilities to climb and build nests.
This is the laws in Sweden you must follow. How does it look where you live? Do you have other laws?