I encourage you to go over and read the whole thing, not that she'll register a couple of clicks from us. It's easier than paraphrasing all she wrote.
To summarize, you are the weapon. Use whatever tool you can find to stop an attacker. There's no such thing as a fair fight.
I want to point out, too, that Brigid approaches the defensive use of the knife in a way that is somewhat similar to my view. I love knives, but I don't care much for all the fancy moves as if you were in a duel. It's not a boxing match. It's not a game. There is one reason to use a knife on another human, and that's if your life or an innocent life which you are responsible to defend is in imminent danger. Everything else is bull. If it comes to that, there is, in my opinion, basically one way to use a blade. Brigid states it quite well:
A knife is a stealth/deception weapon. Show it just to threaten, and you've already lost. Have it ready, but hidden, even in your palm, behind your shopping bag, in your pocket, on a shelf above their eye view but within easy ready. Big blades look impressive, but are only marginally more deadly than a small palmed razor knife used with confidence. [bold added by me]
Should I ever have the misfortune to be in such an unlikely life-and-death struggle, my intention would be for an attacker to realize I had a knife only after he had been cut. Remember her words: stealth weapon.
I would take any knife over nothing, but there is a reasonable lower limit on blade length. Of the knives I commonly carry, my barely-legal CRKT Crawford Kasper would be my first choice as it has a bowie-style blade just under four inches in length, plus a good deal of width with a very solid pommel for striking. The other extreme would probably be my Folts Minimalist with its two-inch Wharncliffe blade. I'd figure on being there a while with the little one.
Honestly, around the house, I'm more likely to be carrying the Minimalist, in addition to a friction folder like a stockman, trapper, or one of my SAKs. Of course, a non-locking, traditional folder can be an effective weapon. It has two drawbacks. First, most are not one-hand openers so it's a little more difficult to deploy. The second disadvantage is that a non-locking blade may decide to fold up in the middle of a melee. I would more or less pretend it was a razor and slash. A knife with a larger choil, like a full-size folding trapper, is probably a little safer for stabbing than, say, a SAK. I might get by with the edge up and a thumb on the choil. After all, I'd only consider using it in a desperate situation. I might not worry too much about a cut finger.