Currently, the highest resolution television you can buy in the United States is 1080 lines. That's not because we've hit a physical limit on how many pixels we can pack into a screen. It has more to do with the HDTV standard. While many high-definition televisions can upconvert standard signals to fit the screen's resolution, true high-definition content is necessary for the full experience.
You can get high-definition video from certain cable or satellite services, a Blu-ray or HD-DVD player or one of a dozen other devices like the PlayStation 3 or Xbox 360. This content should look crisper than upconverted standard-definition content on an HDTV. If a manufacturer offered a set that fit more pixels on the screen, you wouldn't really be able to take advantage of it. You need that infrastructure of content providers to see pictures in a higher resolution.
With smaller screens, a higher resolution may be a waste. That's because the number of pixels in HDTVs remains constant no matter the size of the screen. It's a question of pixels versus area. A 720-line HDTV has a picture made up of 1,280 vertical lines by 720 horizontal lines of pixels. That gives a grand total of 921,600 pixels. A 1080-line HDTV has 1,920 vertical lines by 1,080 horizontal lines. That gives us a grand total of 2,073,600 pixels.
In general, all 1080-line HDTVs have the same number of pixels. Whether your television screen's diagonal measures 26 inches (approximately 66 centimeters) or 55 inches (approximately 140 centimeters), it will still show 2,073,600 pixels. The pixels in the 26-inch model will just be smaller.
But do the smaller pixels make the picture better? If you put a 26-inch HDTV with 720-line resolution next to a 26-inch HDTV with 1080 resolution, you may not be able to tell the difference in resolution. The pixels are already so small at 720-line resolution that making them smaller doesn't help.
On larger screens -- 55 inches (approximately 140 centimeters) or larger -- the 1080-pixel resolution is best. That's because the larger screens use bigger pixels. If the pixels are too big, the image will look jagged or blocky. The distance between the screen and your eyes makes a difference, too. The farther away you are, the smoother the picture will appear.