In order to determine the aspect ratio, you need to have a ratio to aim for.
function getHeight(length, ratio) {
var height = ((length)/(Math.sqrt((Math.pow(ratio, 2)+1))));
return Math.round(height);
}
function getWidth(length, ratio) {
var width = ((length)/(Math.sqrt((1)/(Math.pow(ratio, 2)+1))));
return Math.round(width);
}
In this example I use 16:10 since this the typical monitor aspect ratio.
var ratio = (16/10);
var height = getHeight(300,ratio);
var width = getWidth(height,ratio);
console.log(height);
console.log(width);
Results from above would be 147 and 300