medical - How important is the size of an astronaut?

In the early days of crewed space exploration, size and weight of crew was an issue. The original batch of Soviet cosmonauts selected were restricted to 1.75m height and 72kg mass, for example, and US astronauts to 1.80m and 82kg.

As launchers got bigger and spacecraft got more complex (and roomier), these restrictions became less important because the crew represented a progressively smaller share of the mass budget.

For example, in the Mercury program, the astronaut's mass was ~5.8% of the spacecraft mass; in Gemini, the crew was ~4.3%; in Apollo earth-orbit configurations, ~1.2%.

At some point in that progression, it makes sense to start relaxing the astronaut height/mass restrictions. China's space program simply hasn't advanced far enough for them to do so yet.

As for inherent advantages of a larger crew member, all other things being equal, larger people in good health are generally stronger, which can be important during certain EVA operations. In most other criteria a smaller crew member would probably be very slightly preferable primarily due to resource consumption rates.