Thanks Wayne and Adverser for your generous replies. These sound too technical to me at this point of my learning. But I think I get the point of getting concerned about this only in main loudspeakers.
Would you mind going with me through some details a little bit so I can see how the Golden Ratio applies? If for instance I fashion a main speaker to have the dimensions of 15 in wide and 40 in tall:
1. What should the ideal depth be?
2. What should the dimensions of the back panel be?