Waistcoats are made to be worn with a jacket, so they're sized to match; I wear a 48R jacket, so I wear a 48R waistcoat...a 48S or 48L would be too short or too long in the body, just like the jacket would (it's not just sleeve length).
While I realize that they're pipe-band fashionable, I don't like the look; I think it makes you look half-dressed. I'd prefer just to wear a short-sleeve oxford shirt & tie.