This thread is the reason I registered here. My lover is about 7.5 inches in girth and I'm a small girl. I tear every time we have sex. I have been to a vulva dermatologist (they actually exist!!) and she doesn't think it should be happening but nothing she has prescribed for me or tried has been able to prevent this. I was pretty scared that there was something wrong with me but reading above makes me think that this might just be a normal fittage issue.
As far as sex goes, go slowly to start with lots of lube and then afterwards, treat the tears with cortisone cream once a day for a few days, as it heals them up pretty quickly. I tried Vagisil once, and got the most awful irritation from it. I'd be careful of numbing the areas that tear as you'd likely do yourself more damage in the long run. Sometimes I need to give myself a few weeks between sex as the tears start to get worse each time if I have sex more than once a week.
Something I haven't tried but might do is daily internal massage with sweet almond oil. Supposed to make the tissue more flexible...