When measuring width, measure the drapery rod over the window from bracket to bracket.  When deciding the length of your tab top valance, always measure from the top of your rod/pole down to where you want it to hang.   The tab itself should be included in this measurement.  Our standard tabs are 2" wide and 4" tall, so you will want to adjust your rod/pole high enough above the window (typically 8")  to avoid light shining through.