It's not just China. It's NA, China, EU, Japan. The EU and Japanese markets for an on-target Gen 3 would be much larger because of both price and form factor.
The unknown is a question of "How huge?" I'd suggest that after the Gen 3 launch people who hadn't heard of Tesla would learn about them very quickly.
I think this is largely why they are trying to push out all over the world right now as far as access to the Model S. This way more people will be just naturally exposed to the current products which will naturally increase demand all around for the Gen 3 when it is released.
It happened from the Roadster to the MS. Roadsters were heavy in Cali and less so elsewhere but it was there. People who got the roadsters told their friends, many who said, you know, that's really cool, but I would like something cheaper and also more family oriented. Tesla came back with that response.
Now here we have the MS and MX, and people who the MS didn't work for are lining up in droves for the MX (aren't we slated to surpass the pre-orders for what they had for the MS when all was said and done?). You have the natural exposure that is opening up others to the idea and awareness of the product.
By the time Gen3 is released there will be 2,500 Roadsters, at least 170,000 MS, at least 60,000 MX on the road by the end of 2016 (somewhat conservative guesses, the numbers could easily be higher). These will of course be spread all over the world.
They have also said they plan to triple the number of open stores by the end of this year alone, and triple the number of superchargers by the end of this year. (so that should be close to 300 stores and 300 superchargers spread across the globe)
The exposure level of Tesla will be spread far and wide to the point where I don't think they will have any issue at all selling every single Gen3 they can possibly make in 2017/2018. That will then continue to expose Tesla to the population and it will be harder and harder to find people who don't know that Tesla exists and they make pretty cool cars.
If for some reason they are not hitting the exposure levels they feel they need, then they can easily start advertising in order to increase those levels, but I think I agree with the consensus that the pre-orders are going to blown out of the water. Exciting times!