It is mostly about average rainfall and total watershed. The watershed is how much area drains into your pond. I'm pretty sure there are some formulas on the site. You might try a search of "watershed".
Watersheds work both for and against your pond. Too large a watershed area can overwhelm a smaller pond and wash out the dam. To small an area and your pond won't fill. Average rainfall also works in. There was another post here a couple of days ago that gave a great website that shows average monthly rainfall in your city. Here is the link:
http://www.city-data.com/ (courtesy lloydian). Also interesting is this link from Eastland:
http://www.interwet.psu.edu/R1.htm
