Size does not necessarily equal orgasm.
Large penises can be just as ineffective as small ones.
Lay on your back with your legs close together.Have him rub his penis in a downward motion ,starting from your clitoris.
Keep doing this with little if any penetration.When he gets to the vagina,have him go back up to the top of the clitoris and down again,repeat as needed.
Guide him to the right spot.With ample foreplay ,this should do the trick.