We have different laws and recommendations depending what country we are from. We al know rats wants as big as you can give them. But unfortunately, many laws are making it okay to keep them in extremely small spaces.
Would be interesting to hear what your country has as minimum requirements around the rat's cages and care.
In Sweden, you are not allowed to keep rats under 0.18sq.m. Every individual rat should also have at least 0.06sq.m. Weird is that height is not calculated here. Rats like climbing. But the law has only a minimum height put on 30 centimeters.
Same goes for the smallest side. No side is allowed to be under 30 centimeters.