There is no formula for a web page and you have to consider the type of content and what device the viewer is using.
A "standard" width for a conventional webpage could be considered to be 980px with a page height of no more than about 1200px to avoid too much scrolling. Any page that is much higher than this should, at least, have a "back to the top" button...
http://www.iwebformusicians.com/iWeb/Navigation.html
The browser width on personal computers is user adjustable and not everybody wants to fill the whole screen width.
The browser width on mobile devices is fixed and the user only has the option of portrait or landscape orientation.
US letter size is 8.5 inches wide by 11 high (816 x 1056 px) and the European A4 is 8.27 × 11.69 (794 x 1123 px). Either would be a good starting point for an e-zine.
The iPad is 768 px wide in portrait mode but remember that its not the only tablet available. I think Kindle is 600px wide and others are smaller.
Text on wide pages is difficult to read - hence the use of columns in magazines and newspapers.
Designing web pages for mobile devices in general is not possible with iWeb since it can only use a fixed page width. An example of a folding column magazine style layout can be found on this page...
http://www.ezmacwebdesign.com/showcase.html