Hmmm i dont think there's such a thing as too big hahaha!
Well... actually.. maybe 12" i'd consider too big though i've yet to see one that size.. though i have seen my fair share of "gifted" men in my time!
its funny how when me and my bf had our first threesome my bf thought the other guy was big (and the other guy wouldnt have been more than 7" ) ... i remember quietly thinking to myself "oh i've seen bigger!!!"