It depends. I carry a 4" folder on my belt, plus a Leatherman squit in the pocket. My every Day carry backpack has a larger leatherman, and sometimes a 2.5" fixed. My car and truck have 4" folders. The size is determined by law
Now, my "at home" bags have 3 or 4 various fixed blades up to about 6" - some are custom
I keep a spyderco rescue clipped to my driver's door. It's one of the few fully serrated knives I own. I have a leatherman wave in the glove box along with my arc sls. In the console there is a surefire M2. Under the seat is a large first aid kit. I guess my vehicle is my bug out bag. I've never consolidated it into pack form. I hope if there is an emergency I can gather the things I need. I've planned to put together a BOB, but so far I haven't gotten around to it.
I just came up with an excellent test for a knife that might qualify for a BOB, or any knife where your life might depend on it for that matter. I happened to be working on a little project out in the kitchen with a knife that someone gave me a couple years ago. It was a fairly nice medium sized folder, no special brand, marked stainless steel and Japan. Really nice sharp blade though. I'd just folded the knife and was getting ready to carry it and some other materials I'd been working with back to a storage cabinet, when for some reason I dropped the knife right onto the hard ceramic tile floor. Both scales on the knife went flying, immediately disqualifying it from my personal list of potential BOB knives. [img]/ubbthreads/images/graemlins/thumbsdown.gif[/img]