I assume you are referring to the special zoom modes of "Actual Pixels" and "Print Size".
Actual pixels displays the image on your monitor so that 1 image pixel uses exactly 1 monitor pixel. This zoom state allows you to see the maximum amount of detail available in your image, including detail that you will not likely see if you print at common sizes.
Print size (I don’t remember ever seeing "Actual Print Size" anywhere in Photoshop) is Photoshop’s attempt to show you the image at roughly the same dimensions on screen (in inches or centimeters, not pixels) that it will print at.
In order for Print Size to work reasonably well, you need to make sure that the units in the Preferences are set correctly. To check this, do the following:
Edit>Preferences>Units and Rulers…
Look at the number for Screen Resolution. This needs to be set at the pixels per inch for your monitor. In the past, this was commonly thought of as 72 but these days monitors often have PPIs in the 90s or greater. You can calculate this for your monitor by measuring the width of the screen and then divide the pixel width of your monitor by this number. In my case for example, my monitor displays 1920 pixels horizontally and is 20.5 inches wide. That makes my Screen Resolution approx. 94 pixels per inch (1920/20.5).
Once you have the preference set correctly, then when you select "Print Size" the image will appear on-screen at about the same dimensions as it will print, assuming the current document dimensions and resolutions (as shown in Image>Image Size, for example) are maintained through to printing.
For example, if Image>Image Size tells you that you currently have an image that is 8×10 inches, then if you select the zoom mode of "Print Size", the image should be approx. 8×10 inches on your screen.
Actual pixels shows the image at 100%, Print Size should show the image at approximately the size it will print, but the real print size is shown in the Document Size box in the Image size dialog.
