First off, I would say stop looking at the pros and thinking that they are the be-all-end-all of dancing. They are great dancers, hence the title "pro." They can move like I will probably never be able to. They are incredibly fun to watch most of the time. But, if you start your dancing life, aspiring to be just like them, you are setting yourself up for heartache. Watch them and be inspired by them, but don't try to be like them until you've been dancing for a few years.
After a couple of years of dancing you may come to the realization that you don't have what it takes to get to that level, but you can hold your own in the middle or lower levels of amateur competition. And you just might find that you are quite happy with that.
You might also find that competition is not your bag at all and be quite satisfied with the weekly dance parties at your studio and for that you certainly don't need to worry about being a size six (US) and weighing in at 100 pounds.
As for using these as excuses for being nervous about starting lessons, it's time to get over it. Have you ever gone to the local dance studio and joined a group class? Take my word for it, you have nothing to be nervous about. Take a couple of months worth of group classes and you'll be the one that all the new students come to after class to get help with whatever step the teacher taught that night. And you're a size 14/16. Big deal. In most classes you won't be the biggest woman there, if they're anything like the group classes I've been to.
So, it's time to stop worrying about nervousness and confidence (or lack thereof) and get yourself down to the closest dance studio and sign up for your first group class. Then, after a couple of weeks, sign up for another. And so on and so on.
One word of caution, don't let them talk you into any high priced private lessons or series of private lessons until you've been dancing for at least two or three months. By then, you'll begin to see where your talent lies and where your ambitions are leading. And those silver-tongued devils (teachers) will use all kinds of flattery to get you to pull out your pocketbook and write them a hefty check. Tell them I told you not to and if they have a problem with that, they can take it up with me.
Now, get yourself a pair of dancing shoes and get busy.