I use the 28/32 qt (about 16"X24") all the time for my males and smaller adults, I use the IRIS CB-70 size tubs for my larger snakes (about 18"X35")
My racks are Animal plastics CB70 Racks and the smaller ones I got decades ago from the Boaphile that were sized for the rubbermaid 28 qt tubs. (close to this size > http://www.boaphileplastics.com/rhin...sterilite.html )
I like to get my tubs at Vision Products though since their tubs are thicker, heavier duty and have smooth sides and edges so are easier to clean