Something large like a Kershaw Outcast or Becker Brute and something smallish like a Swiss Army knife or multi-tool with a blade. I've almost never found a need for something in between.
The mid-size "bush" knife is popular nowadays, but it doesn't excel at anything when one gets right down to it.
If I could only carry 1 knife for survival purposes, it would have to be a large one. Bear Grylls disagrees with me on this. Oh well.
1. Martindale N0 2 Machete, 18" overall length heavy thick blade at handle that thins a bit toward point
2. CRKT M21 Special Forces, a very good all around knife
3. Gerber Multi tool, what would Macgyver do without one
In my bags: East German AK bayonet, Gerber Suspension Multi tool with bit kit, Schrade Sharp Finger
CRKT M16-14ZSF is my EDC knife that replaced my M16-03 knife recently and I LOVE the double flippers! With tip up carry it has Spyderco wave style opening with the extra flipper. The AUTOLAWKs is also a plus (the -03 was a old enough model it only had a liner lock ). I also have a CRKT Fixed Falcon neck/IWB knife that I'm sure would be on me in a SHFT moment.