1. I carry a pretty even range of 3" to 4". My rotation curently has five 3", three 3.25", five 3.5", and four 4" knives. (Yes, I just laid them out and counted.) I carry all of those fairly regularly, I most often carry a 3.5" or 4", if I had to give one "most most often" answer it would be 3.5" these days.
2. I carry a knife to cut things - boxes, string, fiberglas strapping, packaging - and as a light tool for scraping and other daily tasks. I carry a knife because I grew up a farm boy, and I put a knife in my pocket when I put my pants on. I leave it in my pocket when I take my keys and wallet out at home, because I use it around the house as much as I do outside, maybe more. I even use it, often, in the kitchen for things that do not require a food grade utensil - opening containers, prying things, making holes.
3. I got my first Barlow knife for Christmas from my father at, I think, age six. That was around 1973.
Glad to be able to help.
Nick