I had the same problem with my ex bro.. I'm not huge by anymeans, I'm 7 1/4 fully erect. Hell, you'd laugh at me if you saw me flaccid.. I"m a grower not a show-er (haha, i loved that thread a few months back). My ex is cuban/italian/indian (native american).. The first time we had sex I just could not get it in.. Nope, wasn't happening.. I had to lube up to get in.. Once she started riding it would tighten up so much that I'd lose my erection.. Even on cialis, yep.. See, the more comfortable she gets with you the more she's going to be able to loosen up and relax those pelvic muscles. The best position is to put a pillow under her lower back and prop her vagina up and spread them legs.. She'll really open up and you can probably even touch some cervix..
The best advice I can give you is to just do everything you can to turn her on.. Tease her, kiss her neck, her back, her upper chest, give her massages, whisper in her ear, breath on her neck, tell her she's sexy, do whatever you can do to make her feel comfortable.. Oh and whatever you do don't fuck things up b/c a tight pussy like that is 1 in a million..
The best position for a girl with a tight pussy is to lay her on her back spread her legs get her on the edge of the bed.. have one of your legs on the ground and one on the bed and slide that shit straight down in there.. Feels soooooooooo good.. she'll love you for it..
T-Matt