What brand of patterns have you looked at? I know Simplicity offers a women's sized line and I'm pretty sure that McCalls does as well. Those are the big Two in my town. It's all they sell.
But, if you needed to, you could just buy the largest pattern, measure the peices, minus the seam allowances, and compare that to yourself. If you need more room, add that amount onto the pattern peices when you're cutting the fabric.
If that's too complicated, or you don't trust yourself with the scissors/ruler combo, make larger pattern peices by cutting the pieces on newspaper instead of fabric first, measure those, and then cut on fabric.
It's not as hard as it sounds, but if it's too intimidating, have a sewing-oriented friend/parent/sewing instructor watch over your shoulder and help.