Last week I had 25.75 lbs in my 10 gallon cylindrical igloo cooler. I would not repeat it probably, efficiency went down below 60%, but I got my 15 gallons of wort out of it for 12 gallons in two carboys.

Saw this sizing chart (click on sizing guide tab) on Blichman's site the other day, http://www.blichmannengineering.com/boilermaker/boilermaker.html

It's meant for their Boilermaker pots but I'll bet its similar to the coolers.

when i said it was a bitch, it was just slow run off. i still get almost my normal efficency i think im off by 5%