One civie lady told me it's all about girth.

I was talking about length, but good question

Xoxo

If the question is asked. It is both. But girth is the biggest issue and results in the most number of rejections.

....... I have an approximately average length penis. So, the ladies I see can't get away with padding my ego about my length, so they pad my ego about it's girth.  
The only reason I have measured was this.
The night before a blindfolded and bound BDSM session, Mistress called me and told me to measure my cock's length and girth while hard. She talked me into an erection on the phone, and i measured. She then directed me to not jerk off. She wanted me to show up with a case of blue balls. I followed Mistresses orders.

The reason for the measurements was for her to prepare a grapefruit with an appropriate size hole to fuck my cock with. It was awesome. I had no idea what it was because of the blindfold and my wrists and ankles restrained.

That was such an awesome session.

For those of us petite ladies.  The length is not as much an issue (for myself, anyway) as the girth, ouch!  

I love the feeling of being penetrated, and a good girth hits all the right spot for that. Too long lengthwise can hit the cervix and hurt. Woman are more maleable horizontilly vs vertically, just from a physiological perspective.