Thanks Hook,
What I've decided to do was to increase the diameter of the 3/4" pipe right after my Back Flow preventer to 1" while it's still in the house and go 1" all the way out and into the ground to the 1" SCH40 pipe that will be tied to a manifold for the 1" sprinkler valves I have. This way I can save on losing some PSI while going out to the valves.
Thanks again.
You can bounch back and forth between pipe sizes all you want. You just have to realize that smaller pipe sizes increase pressure losses as the water flows through the smaller pipe as a higher speed. The other thing to consider is that you don't want to have water travelling through pipes at a higher speed of 5 feet per second. For 3/4" pipe, that limits you to 6gpm to 8gpm depending upon what type of pipe.
You can find pressure loss and water speed charts here:
http://www.hunterindustries.com/resources/pdfs/technical/domestic/lit091w.pdf