[рони]

Gudsaf,

var ball = new ball(1,2,3,4);

function ball (A,B,C,D){
    this.a = A;
    this.b = B;
    this.c = C;
    this.size = calcSize.bind(this)(D)

    function calcSize(diameter){
        return (this.a+this.b)*this.c*diameter;     }
}
 alert(ball.size);

может

var ball = new ball(1,2,3,4);

function ball (A,B,C,D){
    this.a = A;
    this.b = B;
    this.c = C;
    this.size = calcSize(D)

    function calcSize(diameter){
        return (A+B)*C*diameter;
    }
}
 alert(ball.size);