I would think the wider the ditch the better if you wanted a bigger pond.
The small pipe I went with I did for a specific reason at the time. As far as the fish go a much larger pipe would have been better. But my thinking (right or wrong) was that the purpose of this pre-sediment pond was to stop sediment. During normal rain events (where the 6" pipe might actually handle the flow) my intention was to keep the water in the pre-sediment pond as long as possible to remove the most sediment. That is the main reason at the time I went with the small pipe. The pre-sediment pond is small enough I can clean it out with my backhoe. The sediment pond is not.
Had I been doing it strictly for the fish benefit I would have either put a big pipe or a bridge or simple expanded the two ponds into one L shaped pond. The small pipe was to aid in sediment retention.
