Jersey,
Depends on the head. If you have a very low head, you may need to go to a 16" pipe or 2 of the 12 inchers. However, if you have anything more than 5 or 6 feet of head, a 12" siphon will move a *tremendous* amount of water. Even a 6' head will produce a very powerful and destructive flow of water. (better have some big rocks at the outlet!)
I have only a 4" siphon on my 1.5 acre pond, with 10 acres of catchment. It would have to rain very hard and steady to catch up with the siphon. That's partly because the deeper the water gets, the faster the siphon flows.
Beware of back water, which effectively reduces the head.
