Yeah, 16 inches on a length of pull is pretty enormous.
Really want you need to do is measure from the inside of your elbow, with your arm bent at 90 degrees, and measure the distance to the middle pad on your trigger finger. Basically that is your length of pull, modified by half an inch depending on seasonal clothing.
If you got scope bit with a 13.5" - it's one of two things. LOP is one of them and the other is not having a good check weld / poor paralax adjustment and being too close to the scope.
Check the measurement as above and see what you come out at. I would be surprised if you have a need for much more than around 14" - even if your over 6 foot tall. It's just not that common in our shop and the longest length of pull I have ever built for was 15".