you can use a 10gpm submersible and put one of these(link below) inline to reduce the flow...if you are only wanting 2-3 gpm i dont think pipe size will matter too much...pipe friction is a factor when you try to push too much water through too small of pipe...i would say 3/4 inch line would run a few hundred feet with only 2-3 gpm just fine...get a roll of the black poly water pipe

http://www.deanbennett.com/dole-flow-control-valves.htm