I am 6' 7". I am not a small fellow.
My biggest blade is a KaBar Short Heavy Bowie. I think the blade is about 7" long.
I haven't really used it for anything- but not because I don't want to use it. I just haven't been camping since I got it, which is what I got it for.
It has a surprisingly sharp edge.
If I could only take one fixed blade with me anywhere, this would be it.
That said, it's not my favorite fixed blade. My favorite is my Mora Swedish Army Knife, about a 4" blade.
If I'm going to be doing any chopping, I'll probably use a hatchet, or maybe a machete. In fact, if I was allowed two knives in the woods, it would be my Mora and an SAK with a saw.
I also have a GLOCK knife. While it is more of a sharpened piece of metal than a knife, it isn't too huge, due to the fairly slim blade. It's a good stabber, and not unncomfortable to carry.
Note- to anyone considering getting a GLOCK knife, don't. Spend $4 more and get a Buck 119 from Wally World.
I have one Machete- it is a Cold Steel Latin Machete, 18", I believe.
I use it for garden work.
No- seriously. I weed with a machete. I hack at the base of the weeds, from an angle, and it loosens up the weeds, and in some cases, actually removed them.
I have a some back problems, and this more comfortable than kneeling, hunkering, or squatting.
I still threw my back out though- I guess I need a longer machete.
30" blade would be good.
As to folders- in a "tactical" style folder, my favorite size would probably be 3.5" blade, like on my Vex.
But, like on my Al Mar, I have nothing against longer blades, like the 4" on my Al Mar, or on the Buck 110 that I will someday get.
I also have several "tactical" knives with 3" blades, and some shorter. I'm probably going to order a Spyderco Ladybug sometime soon. Small blades have their place.
In the realm of Traditional Knives, I really don't have favorite size. I'll use anything from Peanuts to Trappers, comfortably.
Again, with SAKs- I'll use everything from my Classic to the rather large Helmsman.
I even occasionally pocket carry my Leatherman Kick.
Am I crazy, or what?