Finding the right length for a table runner can be the difference between an elegant table setting and one that makes your table seem undersized. Barbara Reilly of the Tournesol company writes that table runners started as a way to protect tablecloths -- essentially a tablecloth for the tablecloth. Nowadays, they're used more as decoration or as fancy place mats, and they tend to be 20 to 30 inches longer than the total length of the table.
A typical table runner hangs down about 10 to 15 inches or so on each side of the table, no matter the shape of the table. When measuring for a table runner, add 20 to 30 inches to the length of the table to get the length of the runner. The width offers more variation, ranging from 1/3 to 1/4 the width of the table. Use the thinner width on a smaller table, especially if the table is round. This results in less wrinkling as the ends drape over the rounded edge.
Table runners should be long enough to drape gracefully over the sides of the table but not so long they interfere with anyone sitting at the runner ends. If you plan on having people sit at those ends, aim for shorter runners so the edges aren't flapping against knees or resting in laps. If you're using the runner with place mats on either side, ensure there's enough room for the place mat to sit comfortably on the table without pushing up against the side of the runner.