Strokemakers were designed to be oversized, building strength and helping correct stroke faults. The hand should fit completely on the paddle with the thumb and first finger near the edge and the pinky should have some space on the outside edge.
The hand being slightly offset allows for fully finishing the stroke and the outer edge water foil builds strength without putting pressure on the joints Age and experience are two factors listed, but hand size in relation to the paddle is the primary way to determine the correct size.
Most swimmers drop down 1 to 2 sizes from their freestyle size for fly/back/breast as well as during taper. Going up a size or two for drills is also very beneficial to emphasize proper technique and to work on maximum distance per stroke.
Size Guides can be found in the image section for products not listed here.