I haven’t looked at Express in a while, but I’m pretty sure you can
enter any dimensions you want if you type directly into the Width and
Height fields on that dialog. That picking list of default sizes is
just a suggestion.
Furthermore, the dimensions you set in the Document Setup and New
Document dialogs are just a suggestion themselves, as you are always
free to change the dimensions of any page in your site using the
Inspector.
It’s a good idea to stick to one width, so your pages don’t “jump
around” when your visitors navigate, but again, that’s just a
suggestion. Height is traditionally “as long as your content”; that’s
the dominant metaphor of the Web.