The measurement of the length of a piece of string wrapped around your penis determines girth (or circumference). The OP asked for width (or diameter) which is determined by measuring the length across the penis. These are sometimes confused, often to hilarious effect; like the guy who says the width of his penis is 6 inches (horse territory) or has a girth of 1.5 inches (chihuahua su[er stud; human super-pencil dick).
Width used to be most commonly used; Kinsey used it in his studies. Until I came onto sites like this, that's the only measurement I heard of. It's easier to comprehend visually since most of us aren't very good at determining girth by sight alone. If you're in the showers, it's easy to guess, "He's about an inch thick". Most of us don't bring a calculator (or string) into the shower room to calculate girth (pi times girth or circumference). The biggest drawback for using width is that all penises aren't a perfect cylinder; but this is only significant if you're trying to calculate volume. Otherwise, it's a good gauge of how much a penis stretches a vagina.
