I have been reevaluating my choices for survival knives lately.
Previously, I've favored smaller blades. When I was in a Civil Air Patrol ground search and rescue unit I happily got by with first a 5" USAF Survival Knife, then later a Schrade Uncle Henry LB7 Bearpaw folding hunter.
However, during the course of some recent firestarting practice I came to appreciate the chopping power of my Becker BK7. It's size made splitting branches easy when used with a baton, plus with its size I was able to chop at branches to chip off wood shavings for fire starting.
If I was absolutely restricted to one knife and one knife only in a survival situation in most of North America, I'd choose the BK7 without second thought. With a 7" blade, it's a good compromise between a small knife and a big chopper. But since this is a preparedness site and we believe in thinking ahead, IMO it's better to include a minimum of two knives in an emergency kit in my environment (Pennsylvania). Specifically, a small to medium sized blade teamed with a larger knife or hatchet. These will complement my everyday carry Victorinox Pioneer Swiss Army Knife.
The problem with relying exclusively on a small blade is that you may need chopping power in an emergency. Something that can cut the limbs off trees to make shelter or obtain firewood, can split kindling for a fire, or cut the legs off a downed deer. You might be able to perform these tasks with a 4" knife but not as easily as with a larger blade. As an alternative to a large knife, one might carry a hatchet or tomahawk for these chopping duties.
Conversely, a large knife is awkward to work with when performing fine cutting tasks, e.g., whittling camp implements from sticks. And like it or not, large knives tend to alarm urbanites and suburbanites who you may need to deal with.
The smallest blade, the SAK in my case, handles fine cutting tasks, doing so much more conveniently than a larger knife. It's small enough to carry in pocket virtually everywhere yet large enough to handle most cutting tasks I run into on a daily basis in an urban environment. The Pioneer has a spear point knife blade, an awl/reamer, a can opener with small screwdriver on the tip, a bottle opener with a larger screwdriver tip and a wire stripping notch. Victorinox makes another variant of the Pioneer, the Farmer, which adds a saw blade that could be useful. One reason I'm partial to the Victorinox Pioneers is that the handle scales are made from aluminum, so the knives are very rugged.
An alternative to a SAK is a multitool of the Leatherman, Gerber Multiplier, or Victorinox Swisstool type. These add a variety of tools to the basic knife blade, including pliers. I prefer the Victorinox multitools as IMHO they are higher in quality than the Leathermans or Gerbers. Multitools are heavier than pocketknives, so they may or may not work for you.
The medium blade handles larger tasks like skinning game or making fuzz sticks for fire starting. This can be a short fixed blade such as a Mora or a robust folder, such as my aforementioned Bearpaw or a Buck 110. For safety's sake, a locking blade is best if you choose a folder, although I've never managed to hurt myself due to the lack of a blade lock in about 30 years of carrying folding knives.
A side note about folders: The ability to open a folding knife with one hand is important, in case you only have one usable hand due to the other hand being either occupied or injured. Folding hunters such as the LB7 have sufficient weight in the handle to allow the blades to be flicked open. You grasp the blade then flick the handle downwards. The downside to this is that it's not as safe as a knife designed to be opened with one hand, such as a Spyderco Endura.
If you prefer a fixed blade over a folder for your medium sized knife, then a Mora makes a great choice. Although inexpensive, they are good quality knives and take a keen edge without being too hard to sharpen.
For a large chopper I have on order an Ontario 12" machete with a knuckle guard. The foot-long blade should work better than the BK7's 7" blade to clear a campsite, split kindling when used with a baton, and would make a formidable defensive weapon should the need arise. If I lived further south I'd go for a full size 18" machete.
Given my druthers I prefer knives made from carbon steel, especially for large knives. Carbon steel knives tend to be (a) easier to sharpen than stainless, and (b) tend to be a bit more rugged. The Ontario is made from 1095 carbon steel, though my Bearpaw's blade is made from stainless. An exception is the stainless used by Victorinox for their Swiss Army Knives. It's on the soft side and doesn't take me long to put on a razor edge using an Arkansas stone. I touched up the edge the other day and it's freaking sharp!
An additional consideration is blade thickness. This isn't as much of a consideration in folders, but some fixed knives come with blades that are thick enough in relation to their width, that the grind angle is such that they're hard to sharpen. The USAF Survival Knife is a good example of a knife with a blade too thick for its width, unless it was hollow ground.
Knives are among the most important items in your survival kit. They should be selected with care to maximize their utility in your environment. Between the SAK, LB7, and machete, I should be able to handle anything that needs cutting in an emergency.