I think most folks probably use 41qts for large females, 32qts for smaller girls and breeder males, and then smaller for their juvvies and babies. I've been using 12-15qts for juvvies, and when the time comes to build a baby rack I will go with even smaller ones. Iris is good, as are Sterilite, Rubbermaid, and just about any others you can find. Just make sure there aren't any sharp edges the snake can hurt itself on, and if there's a slight dip in the top lip of the tub you want to make sure that's not going to create a gap big enough for your snake to get out/snagged once it's slid into the rack.

I searched around until I found tubs of the size and 'make' that I liked, then designed racks around them using precut melamine shelving from Lowes and the heating bits and thermostat from Reptile Basics.