I don't know the answer (for 200 people), but "pig pickin's" are a tradition in eastern NC (where I went to college)...maybe there's some info here regarding pig sizes.
http://whatscookingamerica.net/Pork/PigPickin.htm
ps: quickly looked, and on one of the links in the thread above, it recommends 1 lb/guest, but I'm betting that with all the fixin's, etc. 150 lbs of pig (i.e. 2 - 75 pounders) would suffice...them folks in eastern NC eat a LOT
