5.5 inches is big enough for good sex. 5-5.5 inches is average so most women will be used to and welcome your size. Don’t count yourself out by guessing what others think...go out there with confidence and know that when she sees you erect, you’ll be perfect for her. My penis is also at 5 inches and it hasn't stopped me from having plenty of lovers and being able to satisfy them sexually. One plus is being able to to have lots of sex without having to stop because she is too sore. Anyway, hard..erection is much more important now so start with that (search googIe for ‘getdpx247’). Find her G-spot using your finger, see if it matches the length from your glans to your pelvic bone. Because if it does, then she's in for a nice time as each thrust from you will stimulate both her G-spot and her clit simultaneously.
Try keeping her thighs closer together, especially if you both like doggy style or prone bone.
If she has a bigger bottom, doggy style may need her to bend forwards more to give you better access. I've had success with her straddling the corner of a mattress, and leaning forwards to take her weight on her hands and arms, while she presents her crotch backwards to meet me.
Girl on top and missionary are both good positions, although I have had overenthusiastic partner bruise my penis.