What size, roughly, conforms to the J.Crew collar size 17 in the Thomas Mason shirts? (I think these were slim fit, but I have a large collar size so it worked out for me.) I'm a medium in Ralph Lauren Polos.
Also, what color would work for me? Taking the advice regarding dark colors, I figure blue would be flexible but it'd be boring, so something like purple might work out better. Then again, that might be restrictive if I need to wear the sweater with a tie in the future.