We're pretty much the same size! I'm 5'1" and about 94 lbs. I actually gained a few pounds without trying while following the Primal Blueprint guidelines. I eat a ton of beef, some eggs, bacon, lamb, chicken, coconut and olive oil, and leafy green veggies. Nuts are great for packing on the pounds, and dairy if you can tolerate it. Just eat meat, veggies and fats until you're full and your weight should stabilize.
Hope that helps!