A numnah should basically be as big as your saddle.
Numnahs that are too short do not protect the entire area of the horse's back that's in contact with the saddle. A numnah that is too large can cause friction or even form folds that can impair fit.
In order to determine the correct size of your numnah, it makes sense to measure your saddle before buying and to choose the size based on the measurement from the rear end of the saddle along the spinal canal to the front end. Simply add at least 5 cm to your measurement and you will have the minimum length that your numnah should have.