3/4 for sprinkler systems. They got it wrong. That frost free if brass should let you soldier inside with a pex male fitting. Disassemble first.
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There are a few systems out there of redidential frost free hose bibs. The actual hose bib body is always the same diameter. The difference is in the fitting.
Some have a 1/2" MIP exterior and 1/2" CTS sweat interior. And some have a 3/4" MIP exterior and 1/2" FIP interior.
I always buy the 1/2" MIP version, and screw on a female thread to PEX adapter. I don't ever suggest sweating on these valves, even if disassembly is possible. That's because the valve bodies themselves are soldered, and while they use a high temperature solder, you still run unnecessary risks by sweating on them.
As for a house feed, 3/4" PEX is pretty small. Since PEX is sized by the OD in CTS, and the walls are quite a bit thicker than copper, the IDs are quite a bit smaller. Basically, the ID of 3/4" PEX is only a tiny bit larger than 1/2" copper. Now they claim to get similar flow rates, IF you have straight runs without fittings, but once you use elbows and the like, you really cut into the flow of PEX. Far more than you would with copper. So, the general practice when replacing copper is to size up one step with PEX. i.e. if you have a 3/4" copper supply to the house, a 1" PEX would be the correct replacement.
And yes, sprinkler systems would get a minimum of 3/4".