Taiwan (a.k.a. the "Republic of China") establishes a maximum ratio of representation of one delegate per 500,000 for their National Assembly according to their constitution. This language can be found in Article 26 (in Section III) which states:
One delegate shall be elected from each hsien, municipality, or area of equivalent status. In case its population exceeds 500,000, one additional delegate shall be elected for each additional 500,000.
Of course, a maximum district size of 500,000 is huge, but it is smaller than 434 of 435 Congressional districts in the U.S. Only Wyoming's single congressional district is smaller with a total of 495,304 residents as of the 2000 census. The average congressional district size in the U.S. was 646,952 (as of the 2000 census).