What we are talking about is the size of your document. So what you need is to resize your doc to the size of your print.
Best way to do this is first decide what you want as a print resolution. If you want quality, then 300 dpi is best.
method:
1/ Open a new doc, white, 300 dpi and choose something easy like 3 inch wide if your monitor can display 900 pixels width.
2/ Double click on this background so as to make it editable, accept the default layer name.
3/ Edit Menu>Transform>scale
4/ Now take a ruler, hold it along the width of your doc against your screen and grab a handle at the side. Drag untill it matches your ruler, in our case: it should be 3 inch on your ruler.
5/ Now look either at the toolbar or at the info pallette and read the % at the W.
Remember this, as this is the scaling factor of your monitor/Photoshop setup.
Now everytime you want to print a doc at 300 dpi, you simply have to fill in this % in your navigator pallette to see your image displayed at true printing size.
Apart from this there is the calibration etc, but that is another story, and when you do a little search on this forum, you will find many topics, and many explanations, tips and tricks. 
