It's a very individual thing. The BMI-based categories were designed to cover every single person in the world, and while they do a nice job at helping you look at large populations, they don't really tell you what YOU should weigh. Many athletes are "overweight" and some are even "obese" because they have so much muscle. And some naturally skinny people can't get out of "underweight" even if they try.
Paleo guru Robb Wolf always says that you can tell whether something is working for you based on how you "look, feel, and perform." So if you think you'd look/feel/perform better at 103, go for it! You can always (healthily!) gain some weight back if it turns out you were wrong.
My goal-weight-choosing story: I'm the same height as you, 5'2", and I've got a smallish frame. The first time I lost weight I picked my goal weight kinda randomly, because I've been overweight since I was a kid. I picked 120, and when I got down there, I wasn't happy yet. But then I regained almost all of what I lost.
This time around I've got A LOT more muscle than I did back then, and my goal is to build even more of it (check out Zuzana on
http://bodyrock.tv, I would loooove to look like her). Based on the previous weight loss attempt, my estimate is that I could get very lean at 115 lb. But I don't really know for sure, so I'll just be flexible, and stop when my body looks how I want it to look.