Its the one where they have the "what-if" machine. And, I think bender asks it "what if I was huge robot?"
It goes into them making Zoidberg huge like Bender to fight him, but first they have to trap him. They put a Guinea Pig on the floor as bait, and he eats it. Hahahahahaha
I cant remember what episode or season it was though. Im at work, and my DVDs are at home.