The total floor space is what I go by.
I admit I am weird, but I positively HATE how shallow most tubs are that are used for balls. Now, I am not saying people who use them are doing anything wrong, but for me personally I don't like them one bit.
So, after much research and hunting and trouble, I located suitable tubs with nearly identical floorspace but taller by a bit to make me happy. My adult male and sub adult female tubs are actually drawers made by Sterilite that I removed from the stackable frames and fitted to my rack.
They are roughly 24" X 18" X 9" high instead of the standard 6". My snakes all do well in them, and I like them really well. I recently moved my four biggest breeder females into Sterilite 90 quart tubs. They carry the same floor space as the 70's, but are 12" tall. I really would have preferred something not quite as tall, but given the choice between smooshing them into 6" or building a bigger than planned rack for the 12", I went with 12".
Now, I know there are dozens if not hundreds of people who use the 6" tall tubs and there is nothing wrong with that, but for myself and my snakes, I like the taller tubs.The point I'm aimlessly trying to make is, by going with floor space measurements, I decided that the almost square drawers I use are basically the same as rectangular 41 quart tubs, and that even though my 90 quarts are taller, they have the same floor space as the 70's.
Gale