How big of a pond , how deep and depth of the dam drop on the back side and how fast? There are several threads here and one includes a link to siphon size calculator. IIRC Burger posted that. I will try to find it.

Here is one

http://www.pondboss.com/ubb/ultimatebb.php?ubb=get_topic;f=20;t=001832;p=1

burgermiester posted this link on another thread:
http://cotton.pi.csiro.au/Assets/PDFFiles/WATERpak/WPAp9_11.pdf