Received my Culdaff (single) 3A '40 today. Great great piece, but I'm a 38R suit, and this is a tad small. Probably would have been better suited with a 42, but this one should stretch enough if I want it to. Plus to my eye there's something more acceptable to a slim fitting 3A than a slim fitting 6A.
real question is whether to grab a standard color 6A (probably navy -- yes, just like the Culdaff -- I'm pale as hell and wear almost all dark blues and browns) in a more relaxed 42 fit.