Hi Ed, its all down to personal preference really trying this out and seeing what works for you.. my personal preferance is what might be considered on the short side 3 and a half inches is probably the most i would want 3 being around optimum i have never found need for more cutting edge than that.. but some people prefer to have a blade of 4-5 inches.
I was just going to say that I reckon that a 5" blade is the best all-around size for doing everything. If you have an axe or machete, you can go smaller with no problems, but if you are just packing one knife that has to do everything from slice lunch and carving to splitting wood, the slightly longer blade wins out.
I would rather have the length and ability to hew on a fire board than have to carve the whole thing.
My method for this is based upon my hand size as well.
The handle must fit in my hand comfortably so will be a little longer than my palms breadth. In order to balance both in terms of wight and asthetics I'd probably have the blade more or less the same length.
Folders tend to be longer in the handle (usually an inch) and if it was a dedicated craft/carving knife all that matters is a small point and edge so it'd have a smaller blade.
All that said I don't butcher or skin with my knives and i certainly don't fell trees with them. They are cutters not choppers!
It is all down to preference, as said already.
If you dont know what your preference is yet, then I would suggest starting smaller.
80-90mm (width of most hands) is a good place to start. R Mears suggests anything from 80-120mm. My first fixed blade (after the mora) was the top end of this range. Now I opt for my hand width (85 ish) or even less.
Also, what are you going to use it for?
A general purpose knife doesnt need to be big at all. The only reason you may want it bigger is for butchering. In which case buy a stainless steel, plastic handle trade knife for 15 ish quid.
The Mora and Woodlore are around 4 inches (10cm). I know quite a few experienced people who go for a 3 to 3 1/2 inch blade.
My personal view is that the slightly longer 4" blade is helpful when batoning to split wood. Without an axe this is a pretty essential task, and very often I don't have an axe with me. So for maximum flexibility I would suggest 4 inches. It is also a common length for traditional Scandinavian knives.
I like that my knives double as a small axe. Because of that I usually only carry a saami knife with a 25cm long blade. It is also very suitable for a draw knife. When cleaning fish it is a bit ungainly, so if fishing I commonly carry along a brusletto knife with a blade of about 10cm.
In Norway, it is only legal to carry knives with less than 10cms long blades in public. So if I am moving in a little more crowded spots I usually bring a smaller knife, if I think I'll need it.