I am flying UA to Beijing this July in a 777 and I wanted to check out the seat options in economy for my award ticket. Seatguru is generally the place to do this. Here is my plane for this flight. Looks like 41L is great, as it is green on the site, but the comments indicate potential disagreement. I googled around and found that 41L indeed is great on a Flyertalk thread, but that 44KL might be better for a two person reservation. I originally had 41KL but switched back to 44KL because Christine is unlikely to let me sit in that nice window seat with extra legroom, so we'll get a two person row with extra aisle room.
Here is where it saved me on a 757 flight:
This was the most crucial of all, as 20A-F look great on the internet, exit row and no one in front of you, but turns out people HATE these seats, no legroom AND no recline. Thank goodness I passed on them as this would be a rather long flight.