How to use protoType

It is me again. Sorry, I have too much questions. ^^

If I create a empty MC, and then loading a jpg on it, how to use protoType to adjust it is size? for example:

MovieClip.prototype.size = function () {
mc = this[name].empty;
mc._width = 40;
mc._height = 40;

It doesn’t work, but why and how?