Agreed, it depends on how big you want it to be, how much you want to spend on it, and what you're going to put in it.
Greater depth will add water volume, decrease temperature fluctuations, allow you to keep a larger number of fish, and protect the fish in case of an abnormal high/low temperature change. Deeper will also cost more money, as you will need a larger liner for it, and it will take longer to dig.