At the
Disney store I usually buy tee shirts in a size L even though I usually like an XL because I don't like them tight but they also carry XL which are very roomy. Also at Disney World I really wanted a Tink zip-up sweatshirt that I had my eye on for a few days. I sent my son down to the gift store at Pop to grab it for me and he called and said they only had XXL and I told him to get it anyway. It is very roomy and I probably could have just gotten a large but I really loved it and I don't mind it big. So I think that you should be able to find plenty of things in plus sizes that are cute for a 17-year-old either at the Disney store or at Disney World. I have heard the Disney outlets in Orlando can be hit or miss as far as sizes and styles but they may have something there too.