Are you planning to supply all the pressure with the PTO pump or is there another booster pump at the reel?  If you are going to use one pump go with larger pipe and be sure to use good reaction blocks, continuous air valves at the pump connection, and  kinetic vents at the risers.   You can get away with four inch if you are going to use a booster.  I'm assuming by "small" you mean something like a 75/250.  If you are running 90/300 you might benefit from six inch pipe if you have to go very far.  The company you get the reel from can give you some engineering help.  If the salesman doesn't know, insist that he call someone higher up.  Pipe and installation are too expensive to want to do it twice.  If your ground is level and you want to increase the efficiency of the reel, consider replacing the gun with a boom.  That will drastically reduce the pressure required and improve efficiency from about 65% to around 85%.  You will need to make more runs of narrower width.