It looks like there will be medium sized Duffy and Shellie May plush for sale at Tokyo DisneySea soon. The official Duffy page lists them for sale on February 10th. These plush are 50cm x 68cm. So, about the size of the big ones for sale at DCA and WDW. However, this is the only way to get Shellie May in this size, since she is not for sale in the other Disney parks.
And Shellie May:
Oh, and each costs 10,000 Yen.
Also, in other news, the press announcement for TDS's 10th anniversary confirms that there will be special 10th Anniversary Duffy costumes. They didn't announce how many or when they will be released, but it's confirmation that there will be some.