For AR pistols chambered in 5.56 the question of barrel lengths comes down to 2 things for me, muzzle velocity and intended use. Sorry to hear you don't live in a "free" state and cant have an SBR, when folks tell me they are building an AR pistol I always suggest dropping a Form1 on an SBR, sorry that is not an option for you. Remember that an AR pistol needs to be built on a virgin lower and the lower needs to registered as a pistol and not a rifle. Just FYI.
Shortening the barrel is going to reduce your MV and likewise your ft/lbs of kinetic energy. When first designed the AR-15 and .223's optimal barrel length was determined to be 20", that said look below you will see the corresponding pistol length barrels and MV's to carbine and rifle length.
BBL___ 77 gr__M855 62 gr__XM193 55 gr
7.5"_____2053________2244________2364
10.5"____2363________2639________2755
14.5"____2576________2861________2984
16"_____2669________2938________3075
20"_____2818________3061________3254
Now that you have an idea of the MV reduction of pistol length barrels, the question of intended use comes in. If this is a plinker, than what will be most comfortable and controllable to shoot is your preference. I have seen 10.5" AR pistols shoot out to 300 meters believe or not quite accurately. If this home defense/protection I would go with what will be the easiest for you to maneuver. My preference for AR pistols is no shorter than 10.5 because of the MV reduction drops off a good bit below that. Its all up to you.
May I also suggest that if your state never legalizes SBR's look into the perfectly legal Sig AR pistol brace.