Of course size matters.
I can't imagine any girl wanting to be screwed with a pencil.
However, medical statistics suggest that the average erect penis size is 5.1" to 5.9" long and 3.5" to 3.9" circumference when measured mid shaft. The average vagina depth is only 3" to 4", until the woman is aroused and then it actually grows in length and can stretch to fit whatever is being inserted.
So, it appears that the average penis size is designed to comfortably fit into an average vagina.
Now, work with me here. If a man can do little, if anything about the size of his penis, apart from some obscure form of surgical procedure, he is stuck with what he has. It is not to his detriment, or his credit.
A lady, on the other hand, has a lot of control over the condition of her vagina and in particular, her vaginal muscles.
Next time a lady suggests that your average sized penis is not big enough, try suggesting that she does some pelvic exercises to tighten things up a bit.