I was looking around eBay for italian shirts and I'm having trouble figuring out my neck size. I measured it myself and it's around 15.5, but when I search for shirts with that neck size most of the times the chest size is around 22/23 (my chest is 39"). Should I go down to 15 or keep the 15.5?
Those measurements are taken when the shirt is laid flat, generally from pit to pit. Trying lying down a shirt you own that fits you to ensure that those measurements are appropriate. I am also a size 15.5 and have a 40" chest, so I'd imagine 15.5 should be appropriate for you.