OK, sorry to disappoint, but the "Big Seat -- XXL" from SFO to Iceland wasn't so great. My wife is shorter than me and she liked it, however. Details:
All in all, I can only recommend these seats if you have rather thin- and not-too-long-legs. Still, it might be better than economy, especially for shoulder room, but even so there wasn't that much shoulder space either.
Finally, although I had pre-booked these seats for both directions of my trip, Wow Air randomly booted me to the back of the plane on the flight back, without any warning, since they aren't very good about honoring seat assignments. So don't count on getting your reserved "Big Seat" -- and it's not that "Big" anyway.