What factors influence your choice in hand guard.legnth.

I'm working on new build ...  16  inch barrel ( to save on weight)

I'm looking at and have handled 15 inch guard and feels good.  

I've shot as short as 9.5    

And as long as I train on it what ever legnth I don't see that huge a difference... Yet.  

I haven't competed yet.    But hope to soon.

Longer offers advantages for both grip and bracing on barricades.  Longer allows for you to extend your support arm shorter does not.  With a 16" barrel you are probably going to want 13-15".  If you go with something like a super slim/ lightweight it will not add a noticeable difference in weight.  Lighter weight equals increased recoil so there is a trade off. That's my opinion anyway.

I'd agree with this. Even on my HD rifle I run a longer 15" rail. It allows me to mount lights out front and still be able to grip farther out. When using my 3gun rifle, I like the longer rail for the same reason, but it also allows me to rest the rail on barricades without worrying about putting the barrel against it.

nothing wrong with 15" handguard.  If your short, narrow shoulders, short arms then 12" would be enough for yoru reach.  However the 15" still offers barricade advantages and mounting locations.

Tall, long arms, & wide shoulders get 16.5" handguard with 18" barrel.