I was considering using a rubbermaid bin for my wet/dry but when I looked into it, it was going to be easier and not that much more expensive to do it with glass.
I purchased a basic tank for the sump from my LFS and got them to cut some glass baffles to size for me whilst I was there. I then siliconed it all up, made a drip tray and it works great.
I based mine on this DIY but mine has only one bio-ball chamber:
http://www.piranha-fury.com/information/default.php?id=wet-dry
As for size of the sump, I'd use the biggest size which will fit under your tank(within reason of course), make sure it will hold enough water so that when the power is out and your main tank syphons back into the sump it does not overflow.
