it all depends on where and in what context you are surviving, wilderness survival could be in any environment that we have on earth.

Case in point, jungle survival would see me bring a 1311 or Killa Zilla.

Urban survival in a major city is often determined by what you are willing to carry size wise depending on how stiff the penalties/laws are. Also, modern combat is mostly in cities and soldier will often carry up to 8" blades and tomahawks.

Bugging out would also change things as you would want to move fast and light so something like a Mud Mutt, Pork Shank or Military Overrun 511 would seem like a good choice.

For extreme cases, you can't even carry a steel knife at all due to metal detectors. In this case titanium, flint, ceramic or even polymer knives would be the ticket which would also change length recommendations.

I firmly believe that 7.5" is about the right size for the most utility/least bulk ratio which will be quite useful in almost any environment if it is designed a certain way.

It should be out of 3/16" stock if strength/weight ratio is important as I believe for any given weight you can make a stronger knife out of 3/16" than you can with thinner or thicker stock's. Meaning, if you're only going to carry a certain weight of knife, 3/16" gets you more strength vs. using 1/4" and up.