For images viewed on computer screens, scan resolution merely determines image size. The bottom line is that dpi or ppi means pixels per inch, which means that if you scan 6 inches at 100 dpi (or 1 inch at 600 dpi), you will create 600 pixels, which will display on any screen as 600 pixels in size.
We think of greater resolution as showing more detail, and while that's generally true (within reasonable limits), it's because it makes the image larger. But we are always greatly limited by our output device, and often cannot take advantage of maximum resolution. The images are huge, and our screens are simply not large enough.
If you don't know your screen size, then the Windows - Start - Settings - Control Panel - Display icon - Settings tab will show or change it. Or on the Macintosh, at the Apple Monitor Control Panel. Screen size settings for LCD monitors should match the actual hardware native size.
Popular CRT video screen size settings historically have been:
640x480 or 800x600 pixels for 14 inch monitors
800x600 or 1024x768 pixels fosr 15 inch monitors
1024x768 or 1152x864 pixels for 17 inch monitors
1152x864 or 1280x1024 pixels for 19 inch monitors
Newer LCD monitors are wide screen, and commonly up to 1920x1080 pixels now. Larger pixel heights typically make the text appear smaller, but we can specify larger text size in Windows (which does not affect image sizes).
Our monitor screens show pixels directly. Images are dimensioned in pixels, and screens are dimensioned in pixels, and video systems show pixels directly, one for one. Bottom line, if we have a 500x200 pixel image, scaled to 100 dpi to print 5x2 inches on paper, on the screen it will still simply show as 500x200 pixels. Zoom certainly is a complication (showing different New resampled pixels then, but still one for one), but 100% Actual size is very obvious. The point is, it's very important, because it's how it works, and it's good to know that.
You do need to realize that each of us will see this 500 pixel image width at a different apparent size on our different monitor screens. We don't necessarily see the same thing. On a desktop video screen, we may see the 500 pixel image anywhere between 4 and 8 inches wide, depending on our screen size and settings.
But different browsers can show it differently too.
Most desktop monitors will be in this general ballpark (almost 6 inches), but a 15 inch monitor set to 640x480 pixels may see it at 8 inches wide, or more. A laptop with 1600x1200 pixel display may see it at 4 inches wide, or less. So that's a very wide size range for the same image on different screens, and it shows that there is no concept of exact size in inches on the screen. We don't all see the same size on the screen. Screens show pixels directly as a dot of color, and their size depends on monitor size and resolution. If the screen is set to show 1680x1050 pixels, then it WILL show 1680x1050 pixels.
So it will be quite necessary to forget about inches on the screen, because video only shows pixels, and screens differ in size. The only one correct answer possible about video image size is that every screen will always see 500 pixels as exactly 500 pixels (assuming we view images at 100% Actual size, otherwise it is about other different pixels). Images are dimensioned in pixels, and screens are dimensioned in pixels, and these 500 pixels will fill 500 pixels on any screen, but those same 500 pixels will fill a different area in inches on different size screens.
Inches simply have no meaning on the computer screen, we all see something different. Inches are not defined in the video system. There is no concept of dpi in the video system either. The way video works is that when you set your video settings to say 1024x768 pixels, then that 1024x768 pixels of memory on your video board defines your video system. The programs you use will copy your pixels directly into that 1024x768 pixel video memory. One image pixel goes into one video board pixel memory location, one for one. A 500x200 pixel image fills 500x200 of those 1024x768 pixels. Those 1024x768 pixels are output to your screen, regardless of the size of the glass tube attached. Video is only about those 1024x768 pixels (or whatever the current setting is).
Unfortunately, we frequently hear how 72 dpi or 96 dpi images are somehow important for the video screen, but that's totally wrong. Video simply doesn't work that way. Video systems have no concept of inches or dpi. No matter what dpi value may be stored in your image file (like 300 dpi for printing), your video system totally ignores it, and always just shows the pixels directly. The truth of this should be clearly apparent if you simply watch what it does.
About what we see:
Your photo editor program normally automatically resamples a too-large image to be smaller so it will fit into the program's window size. Then we only see the smaller copy on the screen, new different pixels, not the original pixels. It also provides a View or Zoom menu, we can create the copy any size we wish, without affecting the original data. The window title bar will show the size reduction ratio, as warning that this screen copy is not the real image data. For example, the title bar might indicate we are viewing a copy at 33% size, or it might say 1:3 ratio of real size (1:3 is also 33% size). We only see the actual original pixels when it says 100% or 1:1 size.
A second situation is Page Layout programs (like MS Word, Publisher, Acrobat, InDesign, PageMaker, Quark). These handle images differently than a photo editor. The very least a photo editor can create is a one-pixel image, its purpose is to create images. But these page layout programs have the one purpose to design and print paper documents. The very least that a page layout program can create is a blank page, which at minimum specifies a paper page dimension in inches. This is very different, it is totally about that page of paper (but people get confused about this). We add text and images to that document to fill inches of printed paper. Page layout programs necessarily do show our document on the video screen, but what we see is an image replica of that page of paper. It may have other embedded images filling areas on that paper page, which are resampled very small to fit their allotted space in the image of the full page we see on the screen. Again, we have a Zoom menu to show that image of the page any size we wish.
In both of these cases, we only see the smaller image copy on the screen (different new pixels), but we print the larger image data using the original pixels. Both cases provide a View or Zoom menu, we can show the images at any size we wish on the screen without affecting the original data. The point is that these are not exceptions, because the video system shows these new resampled pixels in the only way it can, directly, one for one. However, the new image size in pixels is not normally specified to us. Every dimension number we see still pertains to the original size data, (or the size on printed paper), but that is not necessarily about the image pixels we see on the screen (unless we view 100% Actual size).
It also means that when you want to evaluate your image critically, be sure to view the image at 100% Actual size (even if you must scroll around on it), so you are seeing the genuine image pixels that will print, and not a rough resampled temporary copy.
The screen is typically larger than our photographs, so enlargement is often used to show a snapshot photo. We often scan at higher resolutions to fill more of the screen. When we increase scan resolution, we get more pixels, so it increases the image size. But a little goes a long way, and there's no advantage in wrestling with overly huge images just to discard most of the pixels when we display them. So don't scan at 300 dpi or 600 dpi when there's no purpose for it.
How to do that?