It depends. I'm in NYC, but it will vary by state. Some places will sell bottles in 16 oz, some will pride themselves on selling an actual "pint", but you usually get 12-16 oz when you order a beer. Depends if its on tap or bottle/can.
edit to follow up: you do usually just order the drink by name, and the board behind the bar will list if it's bottle or tap, so you know how much you're getting at that point. One of the only places I've actually had serious mugs of beer are Japanese places, actually.