"Dude, personal space. Get out of my bubble." 

Average person's bubble is about 2 feet. Each person having a 2 foot bubble comes out about about 4 feet.

Not that difficult. 

3-10 feet is normal conversation space, to not feel too far away, and hold a normal conversation, closer than that for people that you know fairly well. Skin to six inches is "intimate" space, meaning someone that you're intimately familiar and comfortable with. This does not mean "sex" but it can be. It can be a best friend, family member, or lover. Semi-intimate is usually about six inches to a foot and a half, someone close, a confidant, best friend, or sometimes with something interspersed... or if you're talking to each other without being face to face, but at like a 90 degree angle like L. If you ever watch American men, they rarely talk directly facing each other. They stand next to each other like brothers, or equals, sometimes in a line, or on "corners" or at like an angle form each other. Sometimes 90 degrees, sometimes wider. Sometimes they'll narrow it if they want to be more direct or confidental/conspiratorial. 

Body language and spacing often plays a great deal in how we interact with each other and in subtext. Actors go to great pains to master this language, and police, investigators, intelligence and information analysts go to great lengths to be able to understand it's nuances to tell when someone is lying or not giving the correct information. Psychologists, and anthropologists study it as well and there are many published papers on the subject out there. 

Some of it is universal, culture to culture, some of it isn't. *ponders giving a larger primer on it...*