Is the margin on the page or the table cell?

How wide is your landscape page? US Letter is 11 inches so the page margins will eat into the size you can make your flash cards unless you have very narrow page margins.

The column width could be being overridden by the Table width. For instance, if the table is told to be 9 inches wide and you make both columns 5 inches then each column will shrink to 4.5 inches.

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Andrew Lockton
Chrysalis Design, Melbourne Australia