If you stay in Tokyo for week, there is no way you will use up enough train fares to make a Japan Rail Pass worthwhile. It is worthwhile only if you are going to make a trip at least to Kyoto and back.

By the way, if you DO decide to travel to Kyoto, be advised that you MUST buy the exchange coupon for the JR Pass outside of Japan and exchange it for an actual pass at the airport or at a major JR station. It is not for sale in Japan.

To ease your way, I'd suggest buying a Suica or Pasmo card, which is a debit card for use on all trains and subways in the Tokyo-Yokohama area. They are for sale in all train and subway stations, and you can add more money to it if you run low. It won't save you any money, but if you're familiar with the Oyster Cards in London, it works the same way: just touch it to the turnstile. A ¥5,000 card should see you through a week in Tokyo.

As far as fashions are concerned, the major department stores have large size clothing, but the boutiques may not. They tend to use the odd numbers, like junior sizes in the U.S., and yes, clothing in sizes 9-11 is available.