It's a lot of reading, but if you want to begin to get an understanding of what needs to be done, then you need to read over the web site
www.irrigationtutorial.com
But to get started with a few calculations, check out the charts here (specifically the page for Sch40 PVC:
http://www.hunterindustries.com/resources/pdfs/technical/domestic/lit091w.pdf
One design rule is that water shouldn't travel faster than about 5fps. Based on the chart, you can see that at 10gpm, your water will flow at 6.0 fps in a 3/4" pipe, and 3.7fps in a 1" pipe. So 1" is your minimum size.
The next rule of sizing pipe is that water flowwing through a pipe loses usable water pressure. The chart will further show you that at 10gpm, you will loose 2.4psi for every 100' of pipe. Since you are talking about hundreds of feet, your pressure losses could quickly grow to about 10+ psi. Given that you are starting with 85psi, you've got some room to work with (however, other things will "steal" pressure from you as well). The chart will also show you how small increases in pipe size GREATLY reduce the pressure losses. This is because the pressure loss is a function of the square of the water speed, and water speed is a function of the square of the pipe size. So doubling pipe size cuts pressure losses by a factor of 8. So once you have a clue about all your pressure losses, you can price various pipe sizes and decide which size gives you the most bang for the buck. You're likely to find that you don't really need a 2" line, that 1-1/2" or 1-1/4 will do fine.
The next rule to consider in designing the irrigation system is how far the heads can throw water. The basic rule of thumb that Wet_Boots was giving you is that what ever is the left-over water pressure at the head in psi (after all the losses are considered from the meter, a backflow preventer, valves, and pipe) you can not expect the water to throw farther than that in feet. So if you wind up with only 30psi at the heads, you can't expect a throw with rotors of more than 30 feet. With the pipe sizes and flow you are talking about, 30psi at the heads should be very easy. Likely, once all is said and done, 40psi (and therefore 40') are reasonable expectation. But 50' of throw requires high end rotors AND a minimum of 50psi. That only gives you 15psi to lose through all the equipment listed above. That might or might not be possible. For example, if local building codes require an RPZ backflow preventer, you can expect the RPZ to steal 12-15psi all by itself.
Now practically speaking, my only irrigation experience has been designing and installing an irrigaition system myself for my home. But my best guess at this point is that you need to start by designing for 40psi at the spray heads (which means a maximum throw of 40' with rotors) and work backwards from there.
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