perfectly straight, like an arrow. Enough length to provide a nice stroke, which I'd estimate about 6+ inches, and enough fullness to stimulate the love button, which for me doesnt take a great deal.
About the thickness, some girls would need more if they have a smaller clitoris and/or its a little further away from their vagina. If the girl needs more fullness than the guy can supply, a really good away of working around that is fucking her with you on top, with your face down near her face and her legs pushed up close to her shoulders. This'll get you closer to her clit. Dont do that lean back thing where the dude sits upright and she's laying flat, we dont get much at all out of that. And if we're riding you, dont keep pushing us straight upright so you can look at us....we keep leaning down close to you for a reason, clit stimulation.