Need help making a pull down menu the changes the output of data between SF, LF and Each

So what I need to do is have a pulldown box that contains SF, LF and Each. I want it so when you select square feet there will be two boxes length and width that can be typed in and the square footage calculated. When you select linear feet I want the width box to no longer be able to be typed in and the linear footage calculated. Then when Each is selected I want the length and width boxes both disabled and the total calculation will be just based on quantity.I am using Excel 2013 and any help would be greatly appreciated.