Figure out how many acre feet of water to want to put down for each irrigation session. Then you can size a pump accordingly as to how fast you wish the "Rain" to happen.
Also figure how many hours you want the pump to run without rest. You can move a lot of water in a little time or a lot of time. How fast will you want it delivered, how fast can your soil absorb it, how fast can your well produce it, etc. are all things you might want to include in your calculations.