miha,
What you are asking - if I am not mistaken, is to use %'s in CSS.
It is based on resolution more than monitor size, although generally speaking with current units a 17" monitor may have a higher resolution than the 15.4".
I would do some research on Cascading Style Sheets, and take a look at this example from the css playground, Stu Nicholls is an excellent learning resource on CSS Tips & Tricks.
He provides a layout (as i had linked) entirely based on percentages, therefor on any resolution you look at it on, it SHOULD, in theory look the same or very similar.
The core difference is in text size.
You could also use Javascript to detect the resolution - but i would recommend against it, as many users (like myself) disable javascript as a rule of thumb.
I hope this helps..
