Increase Height in Master Page

I have a Master page and I want to increase the height using my css but it doesnt appear to work.

My css is:

.main
{
    padding: 0px 12px;
    margin: 12px 8px 8px 8px;
    min-height: 420px;
    height: 1000px;
}

If I increase height to 2000px there is no difference when I run the page and some controls at teh bottom of the form are still off the bottom of the page.

Anyone any ideas ?