I enjoy these survival knife threads, because that's exactly what got me into knives in the first place - wilderness adventure.
From my own experience and training, I've learned that having any sort of blade is much better than being blade-less; with that in mind, I also believe that a large knife trumps a small knife and a fixed-blade is better than a folder.
I took a wilderness survival course a couple of times and, in the course, we had to build shelter, make traps/snares, clean and prepare food, start fire, and do all manner of tasks with our knife. The class-issued knife was a Cold Steel Bushman and it worked very well. It definitely gave me an idea of what I do and don't like in a wilderness knife.
Based on those experiences and the training, here's what I like in a wilderness survival knife:
- 7" - 10" fixed blade
- comfortable handle (a subjectuve measure, dependant on the individual)
- a strong tip that doesn't bend/break
- a substantial finger guard; I prefer a single, bottom finger guard
- Micarta or G-10, preferably textured in a manner that will aid traction without rubbing my hands raw
- a pommel or tang extension with which I can pry and hammer
- sufficient thickness to have good lateral strength, at least 3/16" thick at the spine
- carbon steel
- quality. There are definitely some lower-priced blades that are excellent survival knives; there are even more expensive baldes that are excellent survival knives

Bottom line, something that is going to work and keep on working
These are my preferences. I believe that a large knife, while cumbersome, can be used for finer tasks; however, a smaller knife isn't going to do as well with chopping and batoning.
I think that ras made a good point about there being a difference between wilderness and everyday survival. It is true that, around town, basically no one will have a large fixed blade on them. In lieu of this, we all (here, at least

) carry a folding knife. However, I know that, if I was trapped in a burning car or had to smash my way out of a building or ... I'd prefer having a fixed blade knife on me. I do live in a state where this is legal, so that's not a problem.
Blade magazine's most recent edition had a good article on EDC survival knives. The author of that article believes that any fixed EDC should be no larger than 8" overall, for ease of comfort and concealability. THis means that, around town, perhaps the ideal knife is a fixed-blade knife with a 3.75" balde and a 4" handle, worn somewhere that is both comfortable and accessible.
Another consideration is traveling. Whenever I leave town, I like to have my wilderness blade with me - you never know when car problems, bad weather, or any other type of circumstance will necessitate that a traveller fend for him/herself, for a while. When I travel, I take my basic wilderness gear with me - 550 cord, iodine water disinfectant, duct tape, small 1st aid kit, signal mirror and whistle, a smallish tarp, a metal match, a portable knife sharpener (well, gotta buy one of these soon

) a small camp shovel, and a few other things. The kit isn't really that big, but it can cover most any situation. After I rolled my Jeep a couple of months ago

, I once again realized that IT CAN HAPPEN TO ME!