IMO, ideally, it should be as deep as you expect or want yer pond to be. you should examine the soil that will eventually form the bottom of the pond. if no one is entering the excavation it can be quite narrow (the width of a backhoe bucket). a good operator w/ an extend-a-hoe should be able to get at least 15 feet deep. if somebody plans on going in, it needs to be big around enough to do that safely. IIRC, OSHA requirements for unshored trenches is only 4 feet. if you dont shore than it should be benched in terraces. any water put in should just "pond" in the bottom and still be there the next morning. if you have part clay soils (say a silt/clay mix), and water slowly seeps out overnight it could be you just need good compaction of those soils when you build the pond, or you may need additional clay (which could be found on-site based on test pit results) to compact with the native soils at the desired pond depth.

post caveat.....i am no expert, just my 2c.
let some others chime in here.

GSF are people too!