Replacing exterior hose valve, interior connection
Better to use an actual pipe wrench to unscrew the pipe inside from the valve inside while you hold both. A single person doing both parts is better able to balance the forces, and if you unscrew the valve outside you may just screw it off the pipe stub, not the pipe stub from the valve inside as you want. Correct term for "pipe stub" is pipe nipple, my brain belatedly dredges up.

As for "standard length" for frost-free hose bibbs, just get one that is long enough - if that's a little too long, just make a trim block to take up the difference on the outside of the house. A good application for PVC "trim lumber" as it does not rot.

Note that you may be able to skip the whole business by using a drill and screw extractor to remove the broken screw from the valve stem on the present valve. You could remove the packing nut and pull the valve stem to take to someone else for that service (likelihood of just being able to find a replacement valve stem is unfortunately subject to a very wide range of valve makers with incompatible parts, but it's not impossible, just a needle in the haystack.) Your call.