unless your going to shoot a bunch molds at one time i would start out with the 3oz. you can always upgrade to a larger one. but the 6 oz is a fine choice.
i have 3 of bears a 6oz and 2 3oz. and never had a problem. just make sure also you get a pair of gloves. i will make a video shooting with mine.
also have basstackles double injector.