From which size is a country too big to operate as a single country? I think cultural identity is much more important than size, and the Chinese government has put a tremendous effort in culturally unifying the land with great success (and great cost; see Xinjiang, Tibet, Hong Kong, the relationship with Taiwan, loss of local languages and culture). I don’t see that disappearing anytime soon.
A civil war with a stalemate is of course possible (in fact it’s already the reality), but an USSR style collapse in many different countries is just not something I can see happen.