I've decided on 18" or so for carbinizing my FAL. You still get better velocity and thus longer range, but the rifle is still handier than a full 21" FAL or 22" M14.
But, an 18" .308 will still be 38-40" long, depending on model. That's about the same length as a 20.5" barreled M16. So if you think a full length M16 is horridly unweildy, and 18" .308 isn't going to be your bag either.
But, a 16" .308 will still be 36-38", depending on make. (Usually on the lower end only if it has a naked barrel). Handier, but do you think 39" is terribly more unweidly than 37"? I thought about it, myself, and decided that it's not.
Now, a full length rifle is a pain in the butt when you're trying to search a vehicle, but most of us probably won't be doing that.
At any rate, DPMS does make an 18" barreled version of the rifle you're talking about. Probably a better choice for long-ranged shooting, the type that would lend itself to using a bull barreled target rifle.