I'm using a layout for a wide screen that extends past the page setup canvas area. How do you get objects to extend past this if the page size limits this?

Are you noticing the problem in Preview mode? If so, don't go to Preview mode. You could change the page size and limits etcetera, but this won't make a difference when you print since your sizes will be limited by your printer and printer driver. A layout can be just fine in wide screen and display properly in Browse mode but is in effect not printable. I almost never go into Preview mode. Not much point.

I have a 20" monitor, but the widest page I can select in Page Setup is 18", so when I try to add objects past the 18" boundary it won't accept them. So how do I get a 20" page? There is not custom page option.

As far as I know there is no limitation that forces objects to fit in the canvas size. Of course in Preview mode they won't seem to fit. I have several wide screen layouts and they display fine.