…
Now, it is hard to get reliable data on A350 that could be compared as apples to apples… and the example of 787 makes a bit wary about trusting simple numbers sometimes mentioned.

Nevertheless, 559 cm has been mentioned. Which is 10 cm narrower than DC-10…

The Airbus site says the passenger cabin has a maximum width of 220 inches, i.e. 559 cm.

http://www.airbus.com/en/aircraftfamilies/a350/efficiency/in_motion/passenger_cabin.html

The seating flexibility offered in economy class begins with a baseline nine-abreast configuration. Premium economy is created in an eight-abreast arrangement, while 10-abreast seating is available for high-density layouts