Hi guys, I'm a knifemaker, collector, hunter etc. I think the views posted are pretty representative. I've made numerous hunters all under 4inches long in the blade. Even as small as 2inches in the blade for some caping knives. I can number on my fingers how many hunters I've made bigger than that. It seems the prevailing wisdom.
Probably more important than the size of the blade is the SHAPE of the blade for its intended function - skinner, drop point, straight back, trailing point and the shape of the belly. Then equally important is the HANDLE - its length, width and ergonomics.
A drop point hunter with a 3-1/2inch blade and a 4-1/2 to 5inch handle that is well shaped and secure in the hand gives good control and leverage.
Guards and finger grooves are a matter of personal preference. A number of hunters actually bought some of my small double-guarded 4inch fighters because it suited their personal technique. When skinning, caping, quartering, they would let the knife dangle from the fingers balanced across the web of the hand by the double guard.
I don't use guards on my full tang knives and prefer to shape in a dramatic first finger groove and a large hollow that sucks the rest of the fingers in. The butt end needs to be fairly wide so that the little finger can add grip and leverage. Interestingly, note that many of the Scandinavian hunting knives have no guards and generally have quite shapeless handles. They cater to a very specific technique of using the knife.
Hunting knives larger than 4inches tend to get in the way more than anything esle. However large knives can be useful where much more chopping action is required, especially where it is a "camp knife" situation. Sometimes, the hunting knife is the only one available at camp and may be required to limb a few trees, shave fuzz sticks, split firewood. I those cases, a medium-sized blade of 5-6inches with a dropped point, convexed edge and at least 3/16inch thickness might be quite useful for those more heavy duty tasks.
Choose the right knife for the job to suit your technique. Hint - look at Ed Fowler's high performance hunting knives.
Cheers.