Sorry, this is the cage we have our hamster in. We believe the bars are too far apart for mice. We bought a different cage to hold the mice.

You could barely even fit a bunny in here...

Honestly, the U.S. minimum for hamsters is 450 square inches (the Prevue 528 and 40 gallon Breeder are a bit over 600 sq in) and it tends to be higher in other countries. Although this is considered the minimum, I believe that as long as you have plenty of stimulation (a large wheel, climbing toys especially for Syrians, enough bedding to submerge themselves, typically at least 3", a sand bath, plenty of out-of-the-cage time, a perfectly varied diet with insects, seeds, nuts, etc, preferably scatter fed), a smaller enclosure could very well be fine. Though wild hamsters run upwards of 10 miles per night in search of food, domestic hamsters typically only run 5 miles per night on their wheel (which is why a large, quality wheel is required). Cage size also depends on pet size and activity level. Syrian Hamsters, for example, are the largest so I suggest 360 sq in or a 20 gallon Long. Djungarians aka Winter Whites are big for dwarves and very active, so I'd say a 20 Long is good as well. Russian or Campbell's, Chinese and hybrid dwarves are pretty small so 12×24", 240 sq in or a 15 gallon/20 Tall is good. Finally, the roborovski. These may be the smallest but they're the fastest and hardest to tame. I'd suggest a 15/20T as well. I believe that as long as you have the space to fit everything required (at least 3-6" of bedding, a 6.5-12" wheel depending on pet size, toys, tunnels, chews, food, a water bottle etc, the size of the enclosure isn't as important. They still need plenty of space to run, don't get me wrong, but they do most of their running on wheels anyway). Hope this helps!