I haven't flow them but GSTP did a review of the best seats in every class on the A350 (here)

There's a picture in the article but row 45 is recommended as the best in economy - looks like there is a lot of space.

"The best Virgin Atlantic economy seats on the A350 are without a doubt found in rows 45, 52, 53 and 54. Row 45 is the first row of economy and isn’t a “delight” seat, which means it can be acquired even on lower fares. Being in the front means you’ll beat most of the plane through immigration. Row 52 is near the lavatories, but if you don’t mind, you’ll be able to recline sooner during an overnight flight, since you won’t have anyone behind you."