A lot of the evidence about SDL so far has pointed to it being roughly the same size or smaller than Hong Kong Disneyland, I believe.
Disney believes that having a park in Shanghai and one in Hong Kong is equivalent to having a resort in Florida and a resort in California.
Especially because there are 200 million people within two hours driving distance of Shanghai Disneyland.
(I've never visited Disneyland Resort, Tokyo Disney Resort, Disneyland Resort Paris, nor Hong Kong Disneyland Resort, so don't be fooled when I pretend to know what I'm talking about. [But I'm pretty good with the information, if I do say so myself. And you can hit me all you want with Walt Disney World.])