For a website to fit all resolutions, you should use fluid values.
For example, 800px will always be 800px on all screen resolutions. That's 800 small dots across the screen. Pixels are not fluid.
To make a site fluid, you should use values like % and em.
They'll will change depending on the screen resolution.
If you were to set the width of a page to 50%;
- It would be 512px wide on a 1024x769 Resolution
- It would be 640px wide on a 1280x800 Resolution
- It would be 400px wide on an 800x600 Resolution
And so forth.
The Same goes to 'em'. A screen resolution has a certain size for 1em. On all resolutions, 4em will be 4 sizes larger than the pixel size 1em is set for on that resolution.
So, if 1em was set as 8px on an 800x600 Resolution;
- 5em would be 13px
- 12em would be 17px
- 7em would be 14px
And so forth.
If 1em was set as 12px on an 1024x768 Resolution;
- 5em would be 17px
- 12em would be 21px
- 7em would be 19px
And so forth.
As you can see, you should only have to set it once, and on different resolutions, everything should display relative to each other.
Hope this helps,
Ed
NOTE: This is a rough explanation, this will work, but examples are not accurate.