First off, DO NOT USE FLEX PIPE!!!!!! That stuff is absolute garbage, I've never been able to understand why anyone would use it on a vehicle. If your going to remove the cats, and replace it with straight pipe; don't use clamps!! Weld the joints so you don't have leaks under the cab pumping Carbon Monoxide in the cab.
As for what size pipe you'll need, just slip under your truck and measure the OD diameter of the pipe for and aft of the cat(s). And remember with exhaust, you want the front section on the tubing to slip over the Y pipe, and the rear section to slip into the intermediate pipe.
You can get cats much cheaper then 200+, check out thunderbolt cats ; you can get them from $50 a piece IIRC. I hope that helps clears up a few questions for you!