Sorry to hear about the situation with your ex and I can see where your paranoia comes from.
I would definitely look into some self-defense classes for your gf to take in your area. Size doesn't always determine who will win the fight, often times it is the determination to win.
I commend you for the protector role you take over the women in your life and wish you luck with your gf.
"The thing that is really hard, and really amazing, is givnig up on being perfect and beginning the work of becoming yourself." ~ Anna Quindlen
My body is the temple of the Holy Spirit and I glorify God in it by walking in divine healing and divine health. - I Cor. 6:19