When does a knife go from "tool" to "weapon"? Not in terms of legal definitions, which are as varied as they are arbitrary, but in hearts and minds? I shop for knives based on the size of my hand, and how well the grip fits into said hand. I've had more than a few people comment on the size of my knives when I use it to get them out of a bind, but invariably they all kind of shrug and accept "it's not a weapon, it's a pocket knife. It fits my hand comfortably so I have a good grip and can use it safely"
First knife I ever really carried regularly was a S&W Powerglide. It was a reasonable knife, and cool, but the one I got came without the pocket clip it was supposed to, and was slightly obnoxious to carry.
Finally I upgraded to a Cold Steel Ti-Lite. Fantastic knife, great value for money, but unless you're after the switchblade styling, I'd point you to the Counter Point. Same slender blade, available in a slightly smaller size, and it doesn't have the sharp quillions to shred your pockets and thighs.
As I browse for more knives, I only consider those that I know will fit my hand, which is 3.5" to 4.5" of blade. The leading contender remains the Spartan, because I like the little tab for it to auto-deploy when drawn from the pocket. I'm not particularly worried about the actual answers to the question of where people draw the line, but it's interesting to ponder and ask about.