No so big that resizing a window when content is too wide for comfort stops working...

Less facetiously: I wish more pages would give me the choice rather than forcing the content into a fixed-width format unnecessarily, or if they don't mind the extra coding (especially if their design is well thought out enough to make it easy) have a fixed width setting by default with the option to turn it off (I've seen this done by giving the user the option flipping stylesheets via a control at the top of a page) like some support different font size and colour/contrast options.

19 inch