As many people have already pointed out, the 'actual' size of a page depends on the browser, the user's browser preferences, the user's display resolution, the system installed fonts, the display preferences, the user's OS platform, the font aliasing preferences, and the user's browser window size. Occasionally it depends on the phase of the moon, but only at high tide or when pluto is in the House of Aquarius.

HTML doesn't define presentation at all, and HTML with CSS makes really strong suggestions about presentation, but still doesn't define it exactly. (Assuming any browser actually completely and accurately implements the CSS specs, which none do. (Not even counting the spots in the CSS specification that aren't exact enough to definitively render))

The best you're going to be able to do is make a 90% guess. Using an existing rendering engine would be the way to do this, but if you're going to use the result to influence the design as presented to other people, you're probably better off using the IE/Windows HTML rendering engine, as it disagrees with Mozilla on quite a few things and, like it or not, is significantly more used. (And no, I have no idea how you'd do this)