uh, if you think 19 is a skinny neck, you should be able to afford a bespoke shirt from your NFL salary
All jokes aside, though, if you were measured a 19, you are probably a 19. Or maybe you're a 18.75 ? The collar shrinks after a few washings/dryings, so a shirt collar that feels loose now may snug up quite a bit. If you get an 18.5, you run the risk of strangling yourself down the road. Then you would have to reposition the top button, which may or may not ruin the look of the shirt. Of course, if you are not going to wear a tie/unbutton the top, all bets are off, and you can easily go with a smaller size. There's no harm in remeasuring yourself. Especially since you are ordering from Jantzen, you should just follow their instructions and repeat the measurement at least three times to get consistency. Ideally, take a shirt with a collar that you like and measure its size according to their instructions (but tell them that's how you got the size) And tell them to add in an allowance for shrinkage. I've never ordered from Jantzen, but from what I hear, it can be like playing the lottery sometimes, especially with the first shirt. Good luck.