Measurement A is from the horses wither to tail. Measure from the top of the wither to top of the tail. This will give you the EU rug size in cm/inches.
Measurement B is from the centre of the horses chest to the furthest rear point of the rump.
Choose the closest size available, if your horse is between sizes it is normally better to go up a size, unless the horse is particularly narrow chested or petite.