force html page to print constant page size

Hello
im trying to find away to force html page to break the page after or where i define something like every 300px page height
ok .. i know the window.print and the onbeforeprint and onafterprint further more things get complicated cos im getting the html
page generated via some third party application so the total length is unknown only after the page is done generated (note : i can add js to the page before the
html content is generated ..)
so .. in fact im looking for some kind of idea or algorithm or something
thanks alot!

What browsers does this have to work for? There's a series of columns on IE's print templates over at docjs.com

only IE 5.5+ even 6
(its for application web bassed )

Here's a link with all the print template columns. There's six parts in all.

http://www.webreference.com/js/index.html?start=26&page=7