Measure the area where you want to place the runner, with a tape measure.
Draw a sketch of the area on a piece of paper with a pencil. marking the measurements along the sides of the area drawn. Draw a rough image of the ideal size for a runner. Try to leave a border of bare floor between the runner and each surrounding wall.
Estimate the length and width of the rug you drew. If the width of your space is between 3 and 5 feet, a 2 1/2-foot width will leave an appropriate amount of bare floor exposed on either side. Choose a 3-foot width for rooms wider than 5 feet. Choose a length that will leave 1 to 2 feet on either end of the runner. Lengths of runners are commonly available from 6 to 24 feet.