Screen size, what size is right for you?
Non-technical Summary
The optimal screen size for an HDTV is determined by how far your eyes are from the screen. Too small a screen size and it will be hard to see all the details of the picture, too big the picture will appear blury and grainy. Use the chart below to determine the optimal screen size range you should be looking at.

(Min/Max Size is the Screen Size in Inches)
Technical Discussion
HDTV screen sizes range from 19 to 100+ inches, leaving a lot of choices in the middle. HDTV screen size is measured diagonally, from the bottom-left corner to the upper-right corner of the screen; keep this in mind if you need to place your HDTV in an entertainment center that has limited space. All current HDTVs are Widescreen, meaning they are wider than they are all usually with a Width to Height ratio of around 16 to 9 (written 16:9 aspect ratio). A 16:9 aspect ratio HDTV’s screen width will be 1.78 times its height.
The general rule in deciding how big an HDTV to get is determined by how far away you plan on sitting form your HDTV (referred to as the “Viewing Distance”). The equation used to calculate the optimal screen size is your viewing distance divided by 1.5 ( Viewing Distance in Inches/ 1.5 = Optimal Screen Size) — Though a little margin of error is not a big deal.
Ex. 5 ft (60 inches) Viewing Distance / 1.5 = 40 inches
Now that you know what size of HDTV you need, we can move on to selecting your new HDTV’s resolution.