I have some big blades and some smaller ones. Here are some of my thoughts.
Over the years I have observed that bigger blades are usually left behind in favour of the smaller ones. If its not on you it's useless.
I always have a pocket knife, usually a lock blade with pocket clip. If the blade is less than about 3 inches, that too becomes almost useless. (BTW; A large Valencia orange is 3 1/2 inch in diameter)
As the first two inches are for sticking and the two inches close to the hand are for carving, then the standard 4 inches of the common hunting knife found almost all around the world is about right for a belt knife.
I've read that 6 inches for a fighting knife is enough to get through most outdoor clothing and still do the required task. (I haven't stuck anyone yet, so this is theoretical!)
So there you are;
Always have a knife on you, generally a substantial folder.
A belt knife is best between 4 and 6 inches, depending on what you are setting out to do.
Thick blades, coarsely beveled, are also a problem as you can't carve with it.