Well, from my own experience if you have to fit a TV set in a given space (e.g.: between two libraries) you go to the shop with the width and height of that spa

Well, from my own experience if you have to fit a TV set in a given space (e.g.: between two libraries) you go to the shop with the width and height of that space in centimeters and you don't care much about the diagonal in inches. You buy the largest screen that fits.