Well, Airbus quotes the outside length of A350-1000 as 73,9 m. Which is the exact length of B777-300, too. Airbus notably omits cabin inside length, but IIRC so does Boeing. Regarding the fuselage and cabin width, A350XWB is narrower than B777, but both will take 10 abreast coach with some squeezing. The promised range is similar, too.

B777-300ER is flying for years, while A350-1000 is very much a paper plane. Is A350-1000 able to match the whole payload-range capacity of B777-300ER?