Bigger is always an option in my opinion, as long as there aren't lots of open spaces left in the cage
Honestly if it is possible,a 40 gallon breeder tank would be wonderful for a mouse but not quite the best for a dwarf hamster.
If you do buy a tank during the Petco sale,don't get a 55 as they are actually smaller in floor space but taller.A 55 is about 615 square inches,while a 40 breeder is 648 square inches.
However,sides can be cut off tanks and become a square meter or more with 3 tanks connected.It wouldn't be a bad value either,considering you'd pay $120 for 3 tanks if you ever wanted to get a Syrian once the mouse passes but you'd only have to buy 2 more tanks instead.Or,with two tanks you could make it a dwarf hamster cage.