5", 8", 12" Double Ball Arm

As ilustrated above there are lots of combinations you can have for arms. The still photographer usually uses a double arm whether of equal lengths or unequal lengths. The videographer usually uses a single arm length (requires one less clamp).

Arm length is based on what type of photography you intend to do. If you only do macro then you want a small compact arm that will reach your subjects. If you do only wide angle then you want to get your strobes out away from the camera to lessen the backscatter that you get when your strobes light the water in front of the subject you are photographing. If you do both types of photography then you will need to figure the length that will give you the wide angle photography and not be in the way for macro work. The 8" arms are the most popular for both types of pictures.