Just to ease your fears about loose leg straps, from one newbie to another: I'm also a skinny girl, and routinely jumped harnesses that were too big. I tightened everything as tight as it could go, and I always got nervous about it in the plane, even though my instructors checked it and said it was fine.
One thing about leg straps is that they do look looser when you're sitting down than when you're standing up. This is why there's a risk of the leg straps slipping down to your knees if you have a premature deployment in a sit-fly (or so I've read on the forums). However, there's basically no way you can possibly fall out of your harness if you're on your belly and stable when you pull. I did 8 jumps on a harness that was way too loose for me (in retrospect, probably not the best idea...), and it slipped around a bit in freefall, but it wasn't a problem for belly flying. If you're worried about it, just keep your hips forward and legs back, and it should be fine. :) hope that helps!