I have a 48" chest, and a 38-40" waist (just for reference).
I've found that with Nalini jerseys (with "Euro" sizing), even a XXXL is a bit too snug, or on the borderline. They actually make a XXXXL, but they're hard to find.
I also have a Canari jersey (I think that is the name - their logo looks like a little crown), which was quite reasonably-priced, and nicely-made. It uses sizing that is basically the same as US clothes sizing, so that an XL fit me fine.