In reply to:

the user's screen resolutions were set to 800 and 1024, rather than the browsers being that size

Even with a known screen size, the browser canvas's size will vary a lot depending on the user's preferences for how many words they like to a line and at what font size, whether they have other windows on the screen that also need attention (e.g. I like to have two Mozilla windows side-by-side when I'm coding), how much window-manager and browser clutter they have (toolbars, tabbed browsing, sidebar etc.) You really can say very little about the browsing pane size given the screen size (except that the former is rarely larger than the latter).
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Toby Speight
030103016 (80GB Mk2a, blue)
030102806 (0GB Mk2a, blue)