Using an extra 270' of pipe in the supply line will obviously cost extra money, but it will also result in a loss of pressure, because water moving through a pipe loses some pressure. The amount of pressure loss is a function of pipe size an water flow (gpm) (here's where to find a converter tool http://www.irrigationtutorials.com/formulas.htm#sec
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As an example, if you are using Sch40 pipe and designing for a flow of 10gpm, then the 270' of pipe will lose about 6psi in a 1" pipe, 2psi in a 1.25" pipe, and 1psi in a 1.5" pipe.
How ever, RPZ is going to lose about 10-15 psi. As for deciding what size of RPZ to use, you have to look at pressure loss tables and find out which size RPZ loses the least amount of pressure for your desired flow rate.
Two alternate ideas, but you'd have to find out what may or may not be allowed by building and plumbing codes.
#1. You say that the change in elevation in the yard is only 5-6 feet. Is there any reasonable way to install a pressure vacuum breaker about 7 feet in the air at the bottom of the hill? There might be various code that limit the height of the PVB, but regardless of codes, this would only be practical if you use strong metal pipe to and from the PVB, there is something (like a building) to attach the pipe to, and there is some reasonable way to access the PVB for testing without getting on a ladder. While it could be done, it might be so iffy (i.e. must be done just right) that codes may not allow it.
#2. Assuming your local codes allow a double check, you could improve its reliability (and the reliability of your entire irrigation system) by installing a WYE filter up stream of the backflow preventer. Last time I looked at the literature from companies such as Watts, they always show the installation of their backflow preventers being proceeded by a WYE filter. Of course this could only be suggested if you are not injecting anything into the irrigation water (i.e. fertigation).