Let's put that super clear:
There. Is. No. Good. Dick. Size.
There are ladies that prefer 8 inches, and ladies that prefer 4.
Some will not feel it deep enough, some others will say it hurts because it's too deep.
Just...make sure you and your partner find a way to make good use of whatever you've got. Select poses that will set you on the right depth, do magic with other sorts of penetrative and non-penetrative sex, and just listen to each other's bodies.
Also, to everyone who goes with "big dick = alpha" shit, you're both wrong and make it hard to unfuck some guys' minds on what makes them good for sex.