What is the best blade length for a survival knife?

Short answer, 4 to 6″.
Long answer:
One class of interest to those in the survival community is the so called “bush” knife. These are “medium” sized knives, good for most every survival task except chopping, as they don’t have the length, weight or balance to excel at this. The other class of primary interest is the “field”, or “large” sized knives. These excel at chopping. They can be used for most other knife uses, but due to the length, weight and balance, tend to not be convenient for such tasks.
There is a fair amount of controversy over which class is “better” for survival purposes; some experts say that a bush knife is all you need and anything bigger is a waste of space or even “too dangerous” for people in stressful situations; other experts say that a big knife is always better, and can do everything a small knife can do, as well as more. Which side is right? Both have valid points. And neither is completely correct.
So what is considered to be a “field” or “large” knife? As might be expected by its primary task, chopping, the blade is long, usually 9 to 12 inches, and thick (3/16″ to 1/4″), with a lot of weight forward. The edge, at least in the chopping area, may be optimized for chopping; that is, a fairly wide angle grind like you might see on a hatchet. For optimum versatility, the front and back couple inches of the edge could be optimized for slicing; that is, a more narrow angle grind. Serrations should be avoided. In order to get that ‘concentrated weight’ over the chopping edge, a oversize ‘belly’ (the curve between tip and straight edge) is optimal.
The Kukri style blade is an excellent example of this sort of blade. The standard survival optimal blade shape (drop point, spear point or short, straight clip) is most versatile if this is your only knife; if you have multiple knives, then having a more specialized blade shape on your field knife is not only acceptable, but is some cases might even be a good idea.
A “bush” or “medium” knife, then, has a shorter, thinner blade, 4 to 6 inches long and 1/8″ to 3/16″ thick. For maximum versatility it should have bit of belly, but a constant angle grind optimized for its primary tasks, which involve slicing. Drop point is the most versatile tip, while a spear point or short, straight clip point is usually entirely adequate as far as versatility and point strength (thickness) are concerned. Serrations, although they do have their uses, are not as useful in a survival knife. If you really want serrations, a combination blade which has mostly a standard edge with serrations near the grip would be a tolerable compromise.

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