Its so awesome that youre going to get a C and C cage and possibly even another pig! Woohoo! Well first of all, check out this site since it has instructions on how to make, design ideas, photo galleries and size requirements for the amount of pigs:
www.guineapigcages.com
Incase you dont really want to take the time to click that link Ill just post the sizes here
Min-----Preferred-----Cubes
-1 piggy---7.5 sq ft ---(More is better)--- 2x3
-2 piggies ---7.5 sq ft---
10.5 sq ft--- 2x4
-3 piggles ---10.5 sq ft ---
13 sq ft--- 2x5
-4 pigglettes ---13 sq ft ---(More is better)--- 2x6
I would suggest a 2x4. That will give you 10.5sq ft which should be a good amount for two piggies. Your little guy is going to go nuts in that size of a cage! As far as the coroplast, I cant say for sure but I think a 4ft x 8ft sheet is big enough. Im sure one of the rescues can tell you exactly. Whenever you buy the cubes, make sure there are 9 little sqares on a 14in cube, anything bigger can cause problems. I hope this helps!