I have a 25 inch TV at home (I know, mock me…) and I am wondering what size I would need to buy in order to get the same size picture when watching SDTV on a 16:9 HDTV.

(of course I want a really big one, but I want to know what the minimum is to get the same size picture).

So, how do I do the math?

25" is the diagonal of your 4:3 TV, so by Pythagoras its width is 20" and its height 15". You need a 16:9 widescreen TV with a height of 15". The diagonal of a 16:9 rectangle is 18.36", scale it up in the ratio 9:15 and you get 30.6". So you need a 31" widescreen TV.

A standard TV is 4:3. A right triangle with legs of 4" and 3" will have a diagonal of 5". Your 25" diagonal should be 20" wide and 15" high.

Assuming you want the same height, a 15" high screen would be 26.6" wide in 16:9 which gives me a diagonal of 30.5", so I’d go with a 32" screen, that’s pretty popular and will give you a lot of options.