I think that I figured this out. The problem in my case was that I couldn't see the right edge of the table, but this was only because I was in the Print Layout view, which only shows what will appear on the page (more or less). So I changed my view to Web Layout, which then displayed everything, even that which is off the margins.

From there it was introductory stuff. Just grab the lower right corner and resize the table, shrinking it to a size that will fit within the margins. Then switch back to Print Layout view and resize to the precise margins of the page. Problem solved (I think!).