Answer: If you design your web pages properly so that they will fit the browser window regardless of how your visitors have it set then the same page will resize itself appropriately for printing.
If you are using a fixed width layout then stay below 720px width as any wider than that may not fit on the page. The exact amount that will fit across the page depends on the printer, the margin settings in the operating system, and whether standard A4 paper or US letter sized paper (which is slightly wider but a lot shorter) is being used.
If you want more control over print layouts then set up a separate stylesheet with media="print".
This article written by Stephen Chapman, Felgall Pty Ltd.