Not sure what you mean. The physical size on the screen is not the same as print size.
What you you calling size? The actual size for resolution and file size purposes is the height and width in pixels.
To obtain the exact number of your true monitor resolution factor in order to input it in the dialog box shown by Noel in his post above, use the following:
Courtesy of the late John Slate:
John Slate - 3:08pm Jul 15, 05 PST (#7 of 10)
If you want to get exact about displaying print size:
Open a new 500 pixel x 500 pixel file in Photoshop, and set it to 100% (view actual pixels) if it does not come up that way.
Get out a ruler and measure the image on screen in inches.
Divide 500 by the measurement.
This is the resolution of your screen, write it down somewhere. Call it your actual monitor resolution factor.
When I set the canvas size photoshop CS5 opens the images on screen approx 45% than I want them. It is the correct demensions when I print the image but when I save it as jpeg it remains too big.
Can someone tell me how to solve this problem?
On the part about saving as a jpeg,it depends which command you use.
File > Save As... selecting "jpeg" keeps the original document printing resolution. For example 10 x 13 inches at 200 ppi
File > Save for Web selecting "jpeg" would set the resolution to 72 ppi which would change the printed document size to 25 x 36 inches
Solution: Use File > Save As... to keep your jpeg's resolution the same as your original document.