fullscreen scale image proportionally

[3DH]

I'm really close to make a "fullscreen scale image proportionally". Now i know how scale image proporcionally to width, or height. I don't know how i can combine this two scales. For explane first script is for scale width:

sky.height = (stage_width / old_width)*old_height;
sky.scaleX = stage_width / old_width;

And this is for scale height:

sky.width = (stage_height / old_height)*old_width;
sky.scaleY = stage_height / old_height;

I wonna do something like this:

function() {
if (stage_width > stage_height) { 
sky.height = (stage_width / old_width)*old_height;
sky.scaleX = stage_width / old_width;
}
else if (stage_width < stage_height) {
sky.width = (stage_height / old_height)*old_width;
sky.scaleY = stage_height / old_height;
}
}

Idea is really simple. If my web broswer have more width like height, then flash web site use first script. Else use secund script. But i'm not a programmer, so i don't know how i can make this :/

Plz help me