I've got a website, its uses the 960 power grid system, things are fine.

Except the header, the header is simply too tall!

Can you really lessen the entire website size once the user's screen is under or comparable to 1024x768 ?

(same method for you to press Control+ and Control- in IE?)

Technically you could do, however i would advise against using such non-conventional approaches. I'd only use a brief header which will look great in <= 768 and > 768.

Following the document is loaded, you can check (using JavaScript) the customer height from the body and use a style for your <body> element (or simply the header) to create the font size to some number. All of your other dimensions would need to defined in em or percentage models to ensure that the bottom size in the primary container advances to child elements.

IE does offer the non-standard zoom style, for you to affect the <body> should you identify your window dimensions are not big enough.