If you only have 1 bell siphon that is a 1" standpipe, a 2" drain is fine. Be sure to put a cleanout on it. Stuff grows in the pipes and you need to be able to clean them. Like stucco said, you need a little back pressure on the siphon to get it to kick. I put a little trap on mine before it goes into the main drain. Mine drops 6" from the bed, loops up 2", then slopes another 3 back to the pool.