I think pocket knives are like ice cream, some like chocolate, some like vanilla, and then you get fans of rocky road and pistashio mint.
The wonderfull thing about traditional pocket knives is like ice cream, there are so many different patterns, sub patterns, materials, let alone sizes, that there is something for everyone.
For most of my life I was a middle of the road guy. Not too big, not too small. My choice of stockman was the 3 7/8 one. I spent alot of time carrying a scout knife, and I think that was 3 1/2. I had some bigger, like my almost 5 inch Eye-Brand sodbuter, but they were a passing fancy and got replaced with small sodbusters in the 3 and somethng range. But I have never been a suit kind f guy, being a blue collar machinist I were jeans most all the time. I hate getting dressed up. My dad on the other hand was a suit guy. He always had a grey suit on when he went off to work, and he lived his whole life with a 2 7/8 inch peanut. He seemed to adapt to getting everything done with that little knife.
I think clothing worn and life style has alot to do with what the person picks as his carry knife. Suit or work clothes, office or shop. But changing society has alot to do with it as well. The knives that I saw my elders carrying when I was a kid would be scorned by the young adults today. In the 80's Spyderco got the ball rolling big time with the one hand opening knife, in the 90's Benchmade paved the way for the tactical trend, and now its the asisted opener thats hot. For some people today having a knife that you have to actually pull the blade open is like those round plastic things people used to listen to music off of.
Age has a little to do with it. I think alot of young folks want the latest whatever, while an old fart does not really care what others think, he'll use what worked for him way back when. As I have gotten older I have actually went down in size a bit. These days I find a 3 1/4 inch stockman to be almost perfect for my daily use. I have my dad's peanut, and while I like it for sentimental reasons, it can be just a tad too small sometimes. Not often, just once in a while. The 3 1/4 to 3 1/2 seem to be the overall most usefull pocket knife for me. But then I like plain ice cream too, without the nuts and exotic flavors.