You are confused. Video resolution, either analog or digital, is based on the absolute number of lines, not the lines per inch. Standard definition is 480i at a ratio of 4:3. Enhanced definition is 480p at a ratio of either 4:3 or 16:9. In either case, the 480 is 480 whether is it spread out over a screen has a diagonal measure of 12 inches or 12 feet. High definition may be either 720p or 1080i. Again the 720p has 720 lines, whether it is spread over a screen with diagonal measure or 5 inches or 75 inches. The same goes for 1080i. As a general proposition, you cannot adjust the number of scan lines on a TV monitor. On most modern computer monitors, you can. This is, however, not recommended for fixed-pixel displays.
The old standard resolution for Mac monitors was 72 dpi (dots/inch). The Windows standard was 96 dpi. Improvements in technology have rendered those old standards inconvenient--not so much obsolete as inconvenient for a number of reasons. The user may adjust the actual resolution of a monitor to suit his/her needs. At any rate, computer monitors generally have much higher pixel densities than TV monitors. At any point in time, the pixel densities of computer monitors tend to be relatively constant over the spectrum of monitor sizes. Computer monitors tend to display more pixels as they get larger. They have no defined maximum. TV monitors tend display the same number of pixels as they get larger. They have a defined maximum of 1920 x 1080 pixels.
Video content tends to look best on a computer monitor when it displays one pixel of content on one pixel of the monitor. At this 1:1 ratio, my 23" Apple Cinema Display can display 16:9 1080i HD content at full resolution with space left over at the top and bottom. At 1:1, widescreen DVD occupies less than one quarter of the screen. The DVD appears crisp, clear, and small. To get a larger image, you need to be careful. At 1:4, one content pixel will occupy 4 monitor pixels. You will see blockiness in the picture when you sit close to the monitor. At nonintegral ratios, such as those produced on most monitors when you display a DVD at full-screen, monitor pixels may have portions of several content pixels. That may not be so pleasant to watch up close.