I am large and have an a few girls who were extra tight. The right trick that I found was foreplay. On top of relaxing/turning them on with foreplay, make sure you finger them. Start with one finger and make sure to wiggle it and not just go in and out. get her relaxed and wet enough for two fingers (finding her g-spot really helps here) with two fingers as well as wiggling them gently, you also gently spread your fingers apart inside her, slowly stretching her out, when she's relaxed enough add a third finger and repeat. if you know what you are doing you can get her stretching out enough and wet enough so she doesn't get hurt.