20 inches is just fine...One of my buddies is kinda short, he bought the .243 youth model 700 with a 20 inch barrel, I sighted it in for him...It put the Hornady Custom 100grs into a nickle at 100 yards...If you want more speed you can go with the Federal Premium 85gr BTHP, it gets about 3320 out of a 24 inch barrel...

New hunters shouldn't be shooting much past 150 yards or so anyway, any of the 100gr loads will be plenty fast out to that range...