Being a gentleman who has spent extended times exploring the sensual nuances available to my Lover and I in intimate moments, and understanding that you can only 'work with the tools you have', I'll naturally say "Stamina".
By "stamina' though, I do not mean "how long can I pound her in positions X, Y, and Z." Rather, as you put it, the 'Work ethic". Although I do not consider it "work". One, because I am not being paid for it, lol, and two, because it is never a chore.
I just really enjoy the female body, and a Lady's uniqueness of mind and spirit spur me on to discovering more about her unique sensuality and capabilities.
As explained to me by a number of thoughtful and inquisitive Lovers, when they say "size" most often they really mean having that feeling of "fullness" when he is inside her, and having him be able to stimulate her G-Spot, and Deep Spot (area near a Lady's cervix, not the cervix). If he is positioned just right, with slow movements he can also gently stimulate her clitoral area with light pressure from his pubic bone.
The good news for many men who are not in the 7-8-9 inch category is that he can stimulate her G-Spot in almost any position (it only being a couple inches inside her vagina) with an average or sub-average member, and with a strategically placed pillow under her hips/belly, he can greatly stimulate her Deep Spot from either Missionary or Lazy Dog.
Those are just a few examples of taking 'Size' (length) out of the equation when bringing pleasure to a Lady.
The good news too is that if by "size" a Lady really means "feeling full" when he is inside her, well, most men can improve in that area through exercise and techniques to increase their girth. Through stimulation exercises designed to extend the time of full erection, and practicing breathing techniques to maintain control, over time he can add 1/2 inch - 1 inch or more to his girth, which then results in a "fuller feeling" for his Lover.
Finally, circling back to your "work ethic", it pays dividends when a man realizes that often he can bring more pleasure to his Lover with slow and deliberate thrusting, subtly changing angles and depth, and using his hands to caress and stimulate and spread her growing sexual energy throughout her body.
The Taoist writings of the East have a lot to offer us Westerners who are interested in experiencing more with our Lover than what we commonly see portrayed in Western porn.
So, lol, my answer would be 'stamina', of mind, heart and body, coupled with an openness of a Lover, far surpasses the 'tape measure' alone.