A common mistake while measuring is to run your measuring tape along the curves of the sofa. That will give you inaccurate readings.
For every dimension of your couch, you need to measure only from one extremity to another.
Height: For the full height, measure from the floor to the top edge of the backside of your sofa. If you want the height of the lower end instead, measure up to the top of the seat cushion.
Width: Measure straight across from outside arm to outside arm, ignoring any curvature of the back.
Multi-piece: For slipcover sets, you need to measure each piece individually as well. Pay extra attention to the backdrop and the arm depth, for this time, you do need to measure the rounded cushions as well to get the right fit.