I'm about 6'2" and 180-185lbs (depending on the day). I have what I would describe as a fairly slim build (I'm a runner, not a lifter), but far from the emaciated-hipster-musician look that you see in some young men. I ordered the 16-36 with ainsley collar (i.e. BB's partial spread collar) and they are pretty much perfect for wearing buttoned up. If I was only ever going to use them unbuttoned (i.e. sans tie), I may even drop down to a 15 or 15.5 neck. I find that the 16 neck is a tad big for wearing unbuttoned, although I still do it anyway. But a slightly smaller collar (with buttondown) would be better for wearing open, IMHO.
Also, it's probably worthwhile pointing out that I have long legs but an average-length torso. It's possible that someone with a very long torso/arms may find the shirttails a little too short (i.e. come untucked too easily). They work very well for me, though.
I find that it's hard to find decent-fitting shirts with long arm sizes. I guess OTR shirtmakers don't tend to cater to orangutans like me.