- start a new document by going to file > new
- set width = 10cm (or, say, 6 inches... you pick) and height, whatever.
- go to view > print size.
- get out a ruler, and measure the width of your document on your screen. for example, let's say you measured 7.5cm (or 4.5 inches).
- divide your document's width by the measured width... for example, 10/7.5 = 1.33 (or 6/4.5 = 1.33).
- go to edit > preferences > units & rulers and you should see an area named new document preset resolutions. under that, the second field is named screen resolution which most probably has a value of 72. multiply this value by the result above, for example, 72 * 1.33 = 95.76. round that off to an integer. in this case, we get 96.
- delete the 72 and type in 96.
from now on, your print size should be correct -- yay!