Do you have experience reading a topographic map? The watershed is all of the area where water on the surface would flow into your pond. For example, take the mid-point of a ridge by your pond, and water on your side will flow to your pond. Surface water past the mid-point will flow into the next valley over and miss your pond.

P.S. I have a planimeter (surface area tool) on my computer. Do you want to send my your exact pond site in a private message? I can give you a quick watershed estimate.