If you stock a spool (bobbin) of light fishline (10 pound mono, 20 pound spectra braid, kevlar thread, etc) in your kit generally, then a kit can be as small as a pistol cartridge. I sold a few of these a few years back:

And have them scattered in my kits where I don't have anything bigger. Heck, a spool of braided, waxed fishline would work for line in a pinch.

As for the uses, get creative. YOu can catch fish, everyone knows that. Most of us know about hooking birds and small mammals. How about as anchors? Several years ago, I hung white board on my door, but rather than drilling holes in the door, I ran some cord over the top of the door, back down the other side, and some small fish hooks into the underside of the door to anchor it. *shrugs* looked odd, but it worked pretty well.