absolute font-size for fonts/margins/graphics

So I specify the page-width and height for a document as 8.5in by 11in.
Now how would you specify:

1. fonts, margins/padding, content height (seems not naturally to specify
those in subfractions of an in/cm)?
All in an absolute; like pixels or points is fine (does it matter which one?
since both are absolute I beleive)...

2. Also if i have an image that when I set it as a background-image on a
block-container pretend.. If it fits, then i should be good, right?
Like I don't need to worry if the server graphics card changes then the
graphic going out of wack with some dependency on resolution (hoping
something like FOP just takes care of things).

Thanks!,
Ari