There does seem to be some preference for using multiples of the 16 x 9 measurements in inches in determining screen sizes. A 55" screen measures 27" (3*9) by 48" (3*16). Likewise, multiply 9 x 16 by 2.5 and you get a 46.4" (23.5" x 40") screen, or by 3.5 and you get a 64.25" diagonal (31.5" x 56"). You'll find many manufacturers tend to make sets that are 46-47", 55-57", and 64-65". I know that there are 40" and 50-51" sets, but the sizes I list above seem to be more common.
I find this information useful in comparing the sizes of the movies being displayed. Because the screen ratios are different, you cannot directly compare the diagonal sizes; you must directly compare either vertical or horizontal measurements to find a direct comparison. Remember that on a 4:3 tv, the vertical, horizontal, and diagonal measurements form a perfect 3-4-5 right triangle, which makes comparisons easy.
For example, my old JVC 36" was about 21.6" x 28.8". My new Panasonic is a 16x9 56" screen, or about 27" x 48". If I want to know how much bigger Superman (2.35:1) is, I compare the horizontal measurements: Old~29", New~48". A little division tells me that my new image is 66% wider (and therefore, 66% taller). Square that difference, and I find that my new screen shows 2.74 times the area as the old. The same results would come from any material that fills both formats horizontally, ie 1.78 and above.
Doing the same thing with a 3:4 (Casablanca) yields different results. This time, we must compare screen heights. Old~21.6, New~27". Divide and we get an image that is 25% taller (and thus 25% wider). Square this and we find the new tv shows 1.56 times as much area for 4X3 material, a much smaller gain than for 16 x 9.
I haven't worked out the math for 1.66 yet, as it's trickier. 1.66 fills a 4:3 screen horizontally, but a 16 x 9 screen vertically, so direct comparisons of screen size won't work. I have only a few 1.66 movies (Robocop, Disney 90's movies, some European films) but it should come out somewhat closer to the widescreen number than the 4:3 number.