how to fit the web page to the screen

Have a look here:
1. Never give height or width in px..unless it is necessary.
2. Never give <table cellpadding="2" ... instead apply padding class from Css.
3. If u are writtnig document.getElementById somewhere in ur Client side fucntion and if u have the <as:Scriptmanager with u then use $get...
i.e instead of document.getElementById("objID"); write $get("objID"); //as $get is browsewr compatible
4. If u have written code to hide < tr > remove that and palce a div in the tr and then show hide that div.. bcoz it does not work in Mozilla.
5. Avoid writing rowspan and colspans.
6. Write the java script function at the end of ur aspx page.
Have a look ate the xhtml standards

refer these links:
How to Make Web Pages Cross-browser

Top 10 Tips:
Cross browser website tips

Browser-Tutorial for knowledege abt screen size& browsers:
Cross browser tutorial

for your scripts refer this:
Designing for browser and platform compatibility

For Old Version browsers i.e IE6 refer this:
1000 Web Design Tips Compatibility with Older Browsers