I have sort of half-scanned this thread so forgive if I get it wrong.
And I will also apologize if I came off somewhat opinionated. (Hate me or love me, I gave up years ago trying to gain favor for the sake of being accepted.)
Push comes to shove, I am a BIG believer in Mr. Bagwell's EDC of a full-sized Bowie.
I do not believe in carrying a folding knife for EDC especially when the primary job will be for self-offense. A folding knife, whether it be a one-hander, OTF Auto, or what have you. Is not as fast as a fixed blade knife for the average Joe (Not including all of the YOU TUBE Commandos who spend every waking hour toying with a butterfly knife.)
The size of blade depends on your stature, but I would not advise anything less than a 9 inch blade, all the way out to 14 inches for guys built like my best friend, who is 6'7" in his socks. Also the length of one's arms (reach) makes a huge difference. If your knuckles drag, you can handle a longer blade.
If you have not read Mr. Bagwell's compilation book, it would be a good idea to do so if you decided to "go there".
As I write this I am remembering a scene for the Sopranos. Tony goes to Uncle Junior's front cafe to talk serious business. Things go bad, and the last thing Junior says to Tony, "Next time you come in here, come heavy or don't come at all."
The rough spot here is this. If you need a tool to open boxes, or slice your sammich in half, then by all means carry a utility driven folding knife. If however you have decided to "CHOOSE" to carry a tool, wherein it will be used to cease the hostilities of a hostile suspect, then by all means, "GO HEAVY".
Here is something you already know. Knives as a tool to subdue a violent suspect, are frowned upon. At least they are here. If you are in an altercation, a knife may be the last thing you (legally) should try to deploy.
Unless, you target the aorta or subclavian artery, or the renal artery etc, it would be difficult to prove to a jury that a knife was a good idea.
Honestly this is the sort of thread that defense attorneys love to use as evidence of past prior mental attitude.
With that in mind, good luck.
If you want an off-duty carry blade...2 words..."BATTLE and PAPPY" come to mind...any of the sheath makers recommended here are great choices, or try this guy, who makes a great IWB sheath for BIG KNIVES.
http://www.rivercitysheaths.com/photo.htm
Shane