An empty column defaults to its own share of the available space. It usually adjusts to just fit the contents when they are provided. If you double-click the table you'll see a properties page where you may be able to set absolute sizes. Have you considered what might happen if you set a size that won't fit on the recipient's display, maybe because their fonts are larger? There are little red markers with a central X which will delete the associated row or or column. Either side of it are tiny arrows that let you insert an additional row or column.