I'm assuming you're talking about the M-9 bayonet, the bayonet for the M16a2 though the M4.
This is what it looks like on the M4. If you have a 16 inch barrel then you can expect the barrel to extend another 1.5 inches.
You're not gonna hurt the m-9.
I have all the bayonets for US rifles from the Krag to the A2. The M-9 (shown) is the most handy. I keep mine in my 4 Wheeler for emergency fence repairs. Wire cutter on this puppy works better then any fencing pliers I've used. Heavy enough to dig with.