It is not necessary for floorspace to be on one level so multilevel cages with a smaller size base are fine. Of course, the bigger the cage the better. Any wire on cages should be half an inch square mesh to prevent escape. Narrow wire mesh may cause broken feet. Wire should be powder coated or stainless steel.
Galvanised wire can poison a rat.
Wooden cages may smell and rats may chew this type of cage. Plastic and glass cages have poor ventilation. Aquariums should have a wire top for ventilation. Poor ventilation can lead to respiratory illness. More Housing Information Bedding Rats should have litter available on the bottom of the cage to absorb smells from urine.