The total width of both curtains should be two to two and a half times the width of the track or pole. To help you, the width of curtains relative to the width of tracks and poles are listed ...
Measure your window for curtains Decide how much of your window and its surrounding wall you want your curtains to cover. Measure from top to bottom and from side to side.
If you intend to pull curtains back with ties of some kind, you need to plan on more length so that the inner edge of the curtain still falls gracefully.