Owners often shop for a mouse cage by height or by how it looks on a shelf, and mice pay for both mistakes. A mouse colony lives on the floor: running, digging, nesting, and sorting out its social order all happen at ground level, mostly after dark. Sizing a cage well means starting from the footprint, then checking the two details that decide whether that footprint is safe and usable: the gaps mice can escape through, and the depth of bedding the base can hold.
Start With the Footprint
For a small group, a starting point of about a 20-gallon-long footprint, roughly 30 by 12 inches of floor, gives mice room to separate sleeping, feeding, and toileting areas instead of stacking them on top of each other. More is always better, and a larger footprint calms many of the squabbles that owners blame on temperament, because subordinate mice can simply move away. Add mice and the footprint should grow with the group; crowding extra animals into the same floor is the fastest route to bar chewing, nest guarding, and stressed, sleepless mice. A long, low tank or a barred cage with a deep solid base both suit this shape better than a tall, narrow enclosure with small shelves.
Floor space only counts if your mice can use all of it, so picture the finished layout before you buy: a solid-surface wheel, at least two hides, a nest area, water, and open running room should fit without any item touching another. Tubes and levels can enrich a good footprint but they do not rescue a small one, since mice cannot live in a tube the way they live on open floor. If the footprint forces hides into corners with no route around them, dominant mice can guard the only path, and the cage is too small in practice whatever its label says.
Bar Spacing and Bedding Depth Decide the Rest
Keep bar spacing no wider than about a quarter inch, because mice squeeze through gaps that look too small, flattening themselves through openings owners would swear were sealed. Young mice and small females are the most likely escape artists, and an escape usually happens at night into a room that is not mouse-proofed. If you choose a tank instead, fit a secure mesh lid that allows airflow and cannot be lifted from inside, and check corners and cable holes with the same quarter-inch rule in mind. Measure the actual spacing on the cage in front of you rather than trusting a product photo, since spacing often widens at doors and corners.
Depth of bedding counts as usable space in a way owners underestimate. Several inches of bedding let mice dig, tunnel, and build covered nests, which is a large share of their natural daily activity, and a deep base also buffers odor and noise. A cage that holds bedding deeply without spilling it at every opening earns its mice more living room than a shallow tray of the same footprint, so favor high solid sides or a tank-style base when you compare two enclosures. Replace bedding in sections rather than all at once where you can, so familiar scent and an established nest survive each clean.
Recording the Setup for Everyone
Once the cage is chosen, the details worth keeping are the footprint, the bar spacing, the bedding depth, and where the wheel, water, and hides sit, because well-meaning helpers rearrange cages during cleans and quietly shrink the usable space. A photo of the correct layout, taken once, settles most arguments about where things go back. Note the group size the footprint was chosen for as well, so nobody adds a new mouse to a cage already at its comfortable limit. Those notes travel well digitally: a profile on KinsLe shared through Family Sharing lets relatives see the measurements and layout before they clean, feed, or shop for the colony.