Is there anything that says what size sai a person should use? I know most competition rules say what size bo or nunchakus should be used according to the persons size, but what about the sai?

Yes. If you hook your thumb under the guard and let the blade hang down against your forearm, the tip should extend at least to the tip of your elbow. Ideally it should be just a hair past the elbow. This is because you will be blocking strikes holding the sai in this position so you don't want any part of the arm unprotected.

Edited March 9, 200719 yr by AikiGuy

Just a little note, you need to make sure of the measurements of the sai. Ask them what the length from the tip to the guard and from the guard to the pommel tip is. The reason for this is that while a weapon might say a specific length like 18 inches, those two measurements will vary by an inch or two from place to place. It'll still be 18 inches but you may find yourself with a sai with too short a blade and too long a handle or vice versa.

Thanks for the information.