Generally, they give "default" nozzle sizes. Example would be 2gpm for 1/4 circ. 3gpm for 1/2 circ. and so on.
But most of the time, it depends on your pressure and flow. So choose the best nozzle for the application. Less pressure/flow? Then use a 2gpm nozzle minimum in each head with a longer run time. A lot of pressure and flow? Use 3-5 gpm nozzles in each head with short run time.But beware of runoff with high flow nozzles, and winds affecting smaller flow nozzles. Try not to go lower than 2gpm for each head so the wind doesn't affect it much. Try to break up your zones before going smaller...
I'm no expert..........YET!
I just like to suggest things and learn...
See what the pro's have to say first.