As any woman who is being honest (and not just into straight hydraulics) will tell you: the best lovers have both physical skills and the ability to create the environment in which she can let herself go. Once you’ve proven yourself worthy in the physical realm (and consistently deliver that) it truly doesn’t matter if you’re subpar from time to time, they always are imprinted with “he’s the bomb”, and remember all encounters as amazing.
Yes, skill is important and can be a massive differentiator in most situations but given the number of women who have stayed in relationships with subpar sex for decades speaks to the importance or other emotional factors.
It’s layered, emotional and physical. If you can pull of things like making her orgasm by licking her nipples alone or just penetration (that instant you’re in her completely and she orgasms), or just being inside her without movement just looking in her eyes and she cuts one loose, you know you’ve hit the holy grail.
Size is truly only relevant in a very few situations, as long as you’re at least average. It doesn’t hurt to be above average as it’s a force multiplier, but the emphasis many men put on size is truly out of proportion, so to speak.