For the curtain rod:
If you have a curtain rod, measure the length from one end to the other, excluding the decorative finials. For the height, measure from the bottom of the curtain rod to the floor.

For the curtain rail:
If there is a rail, measure the length from one end of the rail to the other, not from one end of the window to the other. For the height, measure from the bottom of the rail to the floor.

* If you do not want the curtain to touch the floor, subtract 3 cm. To just graze the floor, subtract 1 cm. For a puddled look, add 10 cm.