Qian Xuesen was an important figure in the Chinese ballistic missile program after leaving the USA in 1955. The details of his work are not entirely known, but he was a project manager overall, and the lead designer of the CCC-4 ICBM. He was also involved in the early Chinese satellite program. That's about the level of detail one finds in descriptions of his work — a few projects and roles, not much real detail.

The difficulties in knowing "how big of a contribution" all of this are twofold. On the one hand, there is a lot of secrecy — the Chinese don't exactly publish a lot of the history of their nuclear weapons programs (there's some, but not a lot). On the other hand, there is a lot of propaganda — the Chinese decided to make a national hero out of Qian Xuesen, both because of his accomplishments and because he exemplified certain qualities that made him an ideal "state scientist." (This is not uncommon in Communist nations.) So we are lacking a lot of detail, and what we do have is often filtered through a lot of state propaganda.

It seems likely that his role was something like that of Wernher von Braun in the USA, which is to say, he did contribute some specific technical knowledge (he was a world-expert in rockets), but that a lot of his contribution was administrative and advisory. How big of a contribution is that, in the end? It can be exaggerated: nuclear programs are not the work of charismatic individual geniuses, they are collective works typically featuring thousands of people. There were many other scientists working on the Chinese program as well of high quality. The focus on Qian Xuesen is interesting largely because of his past (getting investigated in the USA and effectively deported, then going on to lead another country's weapons program, makes for a good story), but it probably obscures the contributions of other scientists and engineers he worked with.

A nice article on the secrecy/propaganda aspect is Ning Wang, "The Making of an Intellectual Hero: Chinese Narratives of Qian Xuesen," China Quarterly, No. 206 (June 2011), 352-371. Iris Chang's Thread of the Silkworm is a good book about his time in the USA. A good general reference on the Chinese nuclear program is John Wilson Lewis and Xue Litai, China Builds the Bomb (Stanford, California: Stanford University Press: 1988).