This is one of the etiquette questions where the old rule and current practice pull in opposite directions, and the honest answer sits between them.
The traditional logic#
Chinese hosting works on abundance. A host orders more than the table can eat, deliberately, because running out would mean they had been mean. A guest who empties every dish creates the implication that there was not enough — which is precisely the impression the host was spending money to avoid.
Leaving a little in the shared dishes therefore reads as you gave us plenty, which is the compliment.
What has changed#
The Clean Plate Campaign has pushed hard against food waste for over a decade, with genuine cultural traction — restaurants display reminders, and ordering far more than a table can eat now attracts mild disapproval rather than admiration.
The result is a narrower target. Leave a modest amount. Do not leave dishes barely touched.
The practical version#
Take small portions from the shared dishes rather than filling your bowl. You can always take more, and unfinished food in your bowl is more conspicuous than unfinished food in the communal dish.
If you genuinely cannot eat more, say so and stop. Nobody expects a guest to eat past comfort, and continued refusal is understood — expect your host to press once or twice as a matter of form, and decline warmly each time.
Taking leftovers home is completely normal and carries no stigma at all. Asking to pack up the remains of a good dish is unremarkable, including at a fairly formal meal.
Watch your host and follow. This holds for pace, for when to start, and for how much is left at the end — and it is a more reliable guide than any rule you memorise in advance.
For the full set of table conventions, seating, toasting and who pays, see the guide to Chinese etiquette.