[Closed] Safe Frame question

Hi everyone,
As we know,in 3dsmax ,safe frame mode can show the yellow window so that we know the render area,this is base on output height and width,if output height or width changes,the yellow window will change too.
I have a question,for example,there is a scene has an output size,width=640,height=480,the yellow window will shows 640:480 aspect ratio,if I want to let the window shows 640:300,but output size still has width=640,hight=480,is that possible?
My tool has a button can set output height and width to let user view render area ,but sometimes output height or width changes,so I am trying to avoid it,any help?