It depends on your perspective but custom fit is slimmer than classic so whatever size you use on classic I would size down if buying custom fit. That goes for Tees, button downs, polos, etc. but be careful with XS cuz you have to take into account the body length of the shirt too. It may fit...
New Flag Cardi hits the US site:
https://www.ralphlauren.com/men-clothing-sweaters/flag-cotton-blend-cardigan/435150.html?cgid=men-clothing-sweaters&dwvar435150_colorname=Flag%20Multi&webcat=Men%2FClothing%2FSweaters#webcat=men%257Cclothing%257CSweaters&start=1&cgid=men-clothing-sweaters