I like to have this same ability when I'm writing ActionScript to resize objects proportionately too.
Here I use the functions to set the sizes so when I set width, height is changed accordingly.
The same for setting height, the width is changed accordingly.

*Note: I set the inputs to limit the width and height to 500.

Here are two functions for performing proportional resizing by both width and height...

/*---------------------------------------------
Resize proportionately by width.
Returns an Object with new proportionate "width" and "height" properties.
After passing the current width, current height and  a new width.
-----------------------------------------------*/
function constrainSizeToWidth(oldW:Number,oldH:Number,newW:Number):Object
{
	return {width:newW,height:newW / oldW * oldH};
}
 
/*---------------------------------------------
Resize proportionately by height.
Returns an Object with new proportionate "width" and "height" properties.
After passing the current width, current height and  a new height.
-----------------------------------------------*/
function constrainSizeToHeight(oldW:Number,oldH:Number,newH:Number):Object
{
	return {width:newH / oldH * oldW,height:newH};
}