Decide what you want out of Martial Arts and be honest with yourself.

Nearly every person I have ever met that says they do it to defend themselves, to keep fit etc. is really their way of covering up the fact that they want to look cool doing fancy kicks and/or breaking boards and playing with weapons. The others really want to get their hands dirty and kick some ass!! (You get the odd one that is genuinely training for Self defence, but they are few and far between and usually lacking in self confidence).

There is always a halfway house, BJJ and boxing will benefit you as will JKD (although JKD instructors tend to have a base art in which their JKD practice is based). But in all honesty BJJ, boxing and JKD to a lesser extent is a bit like a dry red wine, its not to everybodies taste although it sure will get you drunk!! (bad analogy I know!).

Any way to the point, I wouldn't worry about your weight, you might want to see a doctor to get checked out to make sure you don't have any problems.

As for choosing an art, it's easy to make mistakes early so shop around and see what you want; if it just self defence you will want a school with a reality feel, you will know when you get their (mainly because their will be little or no kicks above the waist). If you see fancy looking stuff like Wu shu, capoeira and TKD they are not as easy to apply to street defence (that was not an invitation to every Wu shu, capoeira and TKD exponent to come up with a 100 examples of how a jump spinning back kick knocked out 7 people at once).

In all honesty go find something you like the look of and then buy "Dog's don't know kung fu: a guide to female self proctection" by Jamie O'keefe http://www.newbreedbooks.co.uk/ that will pretty much sort you out.