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1-inch is the standard minimum size for zone valves.

When speaking of valves and backflow preventers in a typical residential application, 1" vs. 3/4" typically doesn't make much of a difference because there's already opennings much smaller than even a 1/2" pipe the water is having to travel through to get through the device. So usually the desision between a 1" or 3/4" device is usually one of simplisity... which usually means matching your pipe size so you don't have to add step-up/down bushings, etc.

So as long as the specification of the device (such as min/max flow rates and pressure losses at the designed flow rate) meet your requirements, it doesn't matter which size device you use.