Website, DB, & Software Developers

Are screen size - to give you an indication of how much your page is viewable when it first loads.

With HTML the length of the page is theoretically unlimited. You could paste the Oxford Dictionary between the body tags and the page would just automatically resize, lengthways, to accommodate it. Obviously you would want to constrain the width as no-one like scrolling sideways. HTML in the wild runs endlessly down the left hand side of the page unless you box it up in some way.

The browser handles placement of the scroll bars when the page is longer than the browser window. If you are using CSS, make your content div length Auto - it will grow to accommodate whatever you place in it.