I’m thinking about buying a 40 inch widescreen projection tv with a 16:9 aspect ratio.
The question is: When I’m watching standard 4:3 programming, I will have the black bars on the right and left side of the screen(as opposed to the top and bottom when veiwing letterbox format dvd’s on a standard screen), what will be the size of the picture compared to a standard television set. 27"? 32"? Is there a simple formula I could use?
Please help as I figure I’ll be watching standard 4:3 programming for sometime still before the new standard is, well, standardized.
Try this site: http://www.cavecreations.com/tv2.cgi
Type in 40 for the widescreen dimension and anything you want for the 4:3 dimension. On the results page, you will see that a 40" widescreen TV is the equivalent of a 32.7" 4:3 set.
Hope this helps.
P. S.: I’m sure you could figure it out with paper and pencil using the Pythagorean Theorem, but who has that kind of time? 
Well, your TV is 40", 16:9 ratio.
16^2 + 9^2 = 337
sqrt(337) = 18.357…
Scale that up to 40, and we get that your TV is 34.86" x 19.61"
So the vertical part of the screen (and image) is 19.61", the horizontal part of the image will be 4/3 of that, or 26.15", so the image is 26.15" x 19.61".
26.15^2 + 19.61^2 = 1068.37, whose square root is 32.69", which is the (diagonal) size of the image.
I reckon I answered your question, too. 