You will need these basic measurements:

• Width  =  How  wide  is  the  Frame  Depth/Jamb  Depth  
• Length  =  The  distance  between  the  hinge  and  strike  jambs  (this  is  typically  the  door  opening  size  i.e.:  3’0”,  4’0”,  6’0”)  
• Height  =  The  desired  height  coming  off  the  ground  no  greater  that  ¾”  (anything  higher  then  becomes  a  trip  hazard.)