To put what I say in a perspective... I come from northern Finland, where 0F is normal and -40F has sometimes been reached in winter. 4-6 feet thick snow is normal. I have done a few boy scout ski treks in the woods, where we have usually spent a night or two in the snow beside a fire, pretty far from roads and paths.
In the woods I have a 3-4" puukko. I feel that's the optimal length for all of the utility tasks I have for a knife. The tasks are
1) Preparing food. Puukkos are okay for this.
2) Carving wood. Puukkos are designed to do exactly this. I need to carve firestarters from wood when I want to start a fire.
3) Cut misc. things such as string.
Someone who hunts would use it to disembowel/skin, the puukko does this well too. I don't carry a backup knife, but I have always been with others, who have their own puukkos, and you don't really need a knife that often. Never lost or broke one in the woods.
I always have an axe, about 20" long Fiskars (they might be sold as Gerber in the US?) which performs all chopping work.
When used carefully, the axe can (usually) be used to split thin firestarters from wood. Then I don't have to carve those with the knife. Lot less work with the axe.
I don't carry a saw, except when I plan to use up multiple trees as firewood (per night) or build a shelter. A saw helps significantly if multiple trees have to be felled, but is very limited in use compared to the axe.
When in northernmost Finland, it makes sense to dump the axe and instead have a machete-like knife, almost as large as the axe. There are only 1-2" thick little bushes in there (tundra u'know), and that kind of bush is too flexible to cut efficiently with an axe.
No worries with SD here, in the woods you never see another person, and animals just run from you. If SD is a concern to you however, surely an axe/machete is a lot more serious than a "normal" big knife. I won't even start about firearms, which should naturally be choice #1.
So there. I don't find any use for larger knives than 3-4", because the axe or other chopping tool does everything better.
Whoa, long post, but I just gotta tell this story that came to mind.
An old acquaintance's grandfather had made his living as a hunter, hunting alone in the vast woods of northern Finland. He killed more than one *bear* with a single shot .22LR rifle, shot them in the eye. But one bear wouldn't die right away, and charged him. In the end, the man lived, although without half his face. Somehow he had stabbed the weakened bear dead with his puukko. He had a long way home to get help, afterwards.
So.. it's not what you have but how you use it
- Gon