Relative div Height (NOT POSITION)

Hi Guys, i'm trying to convert my table layout to CSS i currently have a 2 main divs(header and main), the header houses the banner, The second div, main is intended to house the rest of the page.

Main div houses 3 further divs, left side page navigation(nav), centre content(content), and right side navigation(rnav).

The main div is important because i want to have margin lines around the 3 smaller divs, however i'm having trouble making the main div's height adjust correctly.

I want to do this because contents isnt a set size, is there a way of the div size being relative to the content of the divs within the main div?

I'm extremely bad at explaining but for a visual example of what i mean see http://hp-h.com/p/amin/sma/html/front.html

my actual stylesheet lives at

http://hp-h.com/p/amin/sma/css/main.css

can anyone give me any pointers on how to do this? it would be most helpful

If your main div does not contain any graphical elements and simply houses the divs > holds them within a set Width only:>

You can set the main div width, and either leave the height blank or 'auto'
hopefully this will adjust when the divs contents expand downwards.

i say hopefully as I have been playing around with this sort of issue over the last week via trial and error, it really depands on all the variables you add.