Physiologically I think we can state with certainty that a very large dick is going to experience more trouble maintaining total erection than a small one for simple reason that more blood has to be corraled from elsewhere in the body to solidify the larger dick. That is not to say that the well endowed cannot get a full hardon but it is more difficult keeping that larger amount of blood welled up for long periods of time without proper stimulation.
I have watched 100's of porn movies with extremely well endowed guys and just average guys and you see near flaccidity during the routine much more often with the huge guys than with the smaller guys. That being said, of course no well endowed guy would want a small dick just because it is easier to keep it rock hard with no little lapses to take a breath and let the rest of the body get nourishment from that well of blood stored temporarily there.
And I speak from experience having been positioned on both sides of this issue and I too would never trade the almost inextinguishable hardness for one that is much bigger but needs a rest every few minutes or hours.
