My old 27" TV is about to die. I came across this site while trying to figure out what size of TV to replace it with.
To find the HDTV screen size (diagonal measure) that gives the same height as your old television, take the screen size of the old TV and multiply by 1.22. So if you want an HDTV with a screen that's the same height as your old 34-inch TV, for example, the chart below shows that you need to buy an HDTV with a 42-inch screen, and that they will both have a height of 20.4 inches.
More size conversions on the screenmath.com website.