Unless the template you're using is static coded in terms of height, you should simply be able to insert a new object near the bottom in a blank area and resize it. Most of the help I saw includes this: "Page size expands automatically to accommodate new content." There might be a limit of 4000 pixels according to some advice out there, but I'd argue that's VERY long and a poorly designed site, not to mention a bandwidth hog. If so, organize your site into pages, which iWeb does well.
I create (almost) all my iweb pages from scratch, and no they don't automatically resize, which is a bit of a pain.
The sizes seems to be coded in, and it's very easy to overlook that when wondering why pages aren't working. To add some extra length, go to Inspector / Page / layout, and change 'Content Width / Length' as appropriate.
Tip: another cause of mysterious page screwups (of pages that don't use the iweb navigation menu template) is accidentally leaving 'display navigation menu' ticked - even if you've already removed all elements from the nav menu area, it still screws up the page display in mysterious ways.