Google has made available a simple tool which provides an interesting insight into what people see when they visit your web pages. You can look in your analytics and see a breakdown of the screen sizes of your visitors (e.g "1024 x 768"), but the size of their browser windows will be substantially smaller than this. They may not be maximised to fill the whole screen, and they'll probably have toolbars and other clutter making the actual view of your site a lot smaller.

The Google Browser Size claims to be able to show "contours" of the percentages of visitors who will be able to see any part of your page. Why don't you try it to see how it works with your website. We'd suggest that anything important falling outside the 80% range should be given attention, as should any messages which cross those lines.