how to make a full screen div, and prevent size to be changed by content?

Or even just:

<div id="full-size">
  Your contents go here
</div>

html,body{ margin:0; padding:0; height:100%; width:100%; }
#full-size{
  height:100%;
  width:100%;
  overflow:hidden; /* or overflow:auto; if you want scrollbars */
}

(html, body can be set to like.. 95%-99% or some such to account for slight inconsistencies in margins, etc.)