The 777-9X cabin will be 16 inches (40 centimeters) wider than the A350-1000, Boeing says, allowing economy-class seat widths up to 18 inches (46 centimeters).
Typical airline seat widths range from 17 to 18.5 inches (43 to 47 centimeters).
Well I'm pretty sure we know what most operators will do in reality - cram as many passengers in with the narrowest possible seats, rather than go towards the larger end of the spectrum.