How to find out what size rack you need?

I want to build a 4 slot 41qt melamine rack around christmas and i was wondering...how do i figure out the length of wood i need for the back, sides, and shelves??? this is the tubs im using: its a sterilite #1960.

http://www.wasserstrom.com/restauran...t_326426#close

Check out this site

DIY reptile racks

I am thinking about building one too! Good luck with your build, get some photos up when its finished! Cheers!