About the size of two regular coffee mugs.
My regular coffee mug is 24 oz...
Of course, some people (notably the ones who print brewing directions on coffee cans - 'for each cup of coffee use 6oz. of water') still think a cup of coffee is 6 oz. That can't even be right in the real world since a 'cup' is 8 oz..
In accordance with those facts, if I'm brewing a cup of coffee I fill the measuring carafe to the 4 cup mark...