I am thinking more than 3 feet deep or the Herons will have a feast at your expense! Even then with the side dropping 3 feet instantaniously, the Herons will stand on the bank and still be able to do some major damage but a small fence at the edge might remidy this. As for size I am not sure how big it would need to be to produce a sustainable population for you. I would imagine it wouldnt need to be too big.