No, it's not something easy. The thing is: a page is not measured in centimeters or inches, so it cannot be A4 or Letter size. The whole idea is wrong. The size of the page depends on how the user looks at it, screen pixel dimensions of user computer and user-selected browser window size and magnification. Pretending it's something else and trying to mimic different behavior would be against the nature of Web.
After all, if you need exact printout, make a PDF file on the fly and offer the user to print it. Don't try to print the page, print your data properly formatted for printing. To prepare such PDF document, use iTextSharp library, see
http://sourceforge.net/projects/itextsharp/[
^].
—SA