Wow! Congrats, but I know you're still in shock. I don't have any direct advice on twins for you, but I had a friend who had twins a few years ago and she was absolutely fine during the entire pregnancy. She was not on bedrest and had a very healthy pregnancy.
I'm sorry you had such a negative experience with the nurse. Hopefully your OB can explain what to expect to you in a little more calm terms. I would do the nutritional counseling just for your own knowledge. If you do have high glucose and/or BP, it can be treated with changes in diet and activity and isn't an automatic bed rest sentence.
I think most twins deliver a little early basically due to lack of room in the womb. But, most are still considered "to term" even if they come a bit early. I'm sure this will also depend on how your appointments go.
My suggestion overal would be to simply treat this as a single pregnancy to get started with. Focus on healthy eating. Here's a great site that explains the good components of a healthy pregnancy diet and also addresses carrying twins: Dr. Brewers Pregnancy Diet. You might also look into attending a Dr. Sears LEAN Expectation workshop series if there is one near you (or you can do some online). It has some wonderful & basic advice on achieving healthy nutrition and general wellness during pregnancy. As for clothes -- we're all going to get bigger . You'll just probably get a little bigger than those of us carrying single babies. I wouldn't worry too much about your size after birth. As long as you have a healthy pregnancy, the weight that you gain during pregnancy will come off fairly easily.
Good luck. I'm sure your head is reeling right now. Hopefully someone can give some better "been there, done that" info for you.