The knife I use has a blade that is about 3" long and I cut everything from boxes to wire with it. It’s made by Blackwater Gear and opens easily just by flicking your wrist.
I’ve also got a few small kershaw knifes that are assisted opening which are pretty sweet.
My SOG Nightvision does not have assisted opening (and I didn’t think most of them did, AND assisted opening is illegal in most places). It has a cam lock, which is very smooth for one handed open/close. I wear it every day and have for 6 years or so. Keeps an edge very well and holds up to a lot of abuse.
http://sogknives.com/store/CFSA-98.html
that’s the exact knife i have and use (it was a gift)
i love it
I think its less than 6" and has to be a fixed blade. Can not be a switch blade.
Also, dont try to bring it on a plane no matter what size it was. I had one on my key ring that was about 1 1/2 inches and airport security flipped shit on me. Lol
my last wallet was made by Case and had a knife in it.
I forgot about it and accidentially took it on a plane the first week of oct. 2001. Very very very glad they didnt find it or I might be in guatanamo now.
wound up picking up a SOG fusion, something like a 3.7" blade. it has a lowrider pocket clip, and with a slight pressure on the thumb stud it flips out with a small snap of the wrist. looks to be nearly flawless build quality too.
because of the surface finish i cant tell for sure, but it sure seams like everything is either laser cut or machined to shape. definantly doesnt have that “we stamp out 10,000,000 of these an hour” look to it
nice knife so far, quite happy with the purchase
http://www.bladehq.com/item--Gerber-06-Automatic-Knife--1471
this is what i carry. I don’t have it on my all the time… but if im going to be outside I’ll definitely have it… and sometimes if im just out n about i do too.
If the Army didn’t issue it to me i’d never have it… $180 and i have a reason to carry a switchblade now since Uncle Sam issued it to me 
i never go anywhere without it, i use it daily and love it!