How big an injury to flaccid penis would create a venous leak? i hit area, no bruising or swelling after, still can get good erections for many min.

No, very unlikely...: What you described implied the hit is low impact and shouldn't be strong enough to cause sufficient injury to flaccid penis leading to venous leak, which is at times mentioned as the cause for erectile dysfunction and is not related with prior injury but with venous defects in its valves and insufficient compression mechanism to occlude the blood leaking out the penis. So, don't worry about it.