There is a common misconception that the correct “resolution” for images viewed on a monitor or the web is 72 ppi. One of the undesirable side-effects of believing this misconception is that it can lead you to get the wrong display of images with the View > Print Size command in your image editing application.
The problem is that View > Print Size depends on your application knowing the correct screen resolution and your screen resolution is probably not 72 ppi.
For example, the screen resolution of my laptop monitor is 147 ppi (1920 pixels over 13 inches). Lets say I have an image that will print 6 x 4.
By default, View > Print Size displays this on my laptop at 2.94 x 1.96. Why? Because Photoshop thinks my screen resolution is 72 ppi (the mythical default number). So, by default, View > Print Size displays images at 72/147 = 49% of real size.
In Photoshop, you can set the screen resolution in Edit > Preferences > Units & Rulers > Screen Resolution. Setting mine to 147 results in View > Print Size correctly displaying the image at 6 x 4 on the screen.
To determine your monitor’s screen resolution, just measure the width of the displayed area in inches and divide that number into the pixel width of your display settings. For example, suppose your display settings are 1600 x 1200 pixels and you measure the width of the displayed area as 13.9 inches. Then your screen resolution is 1600 / 13.9 = 115 ppi. Using that setting in Photoshop will produce the correct image size with the View > Print Size command.
It’s a good idea to verify that you have the right setting by placing a 6″ x 4″ photo up to the screen and comparing. If the image on the screen is smaller, then increase the screen resolution setting. If it is larger, then decrease the screen resolution setting.
NOTE: If you’re using a CRT monitor, be aware that the vertical screen resolution can be different than the horizontal screen resolution. Before you measure your screen resolution, you need to use the monitor’s hardware controls to ensure the display is “square”.