I kept meaning to come back here to post an update. In my first post I said that the bed was too narrow and my left arm kept falling off the bed. Well, on my return flight I managed to position myself so that my arm was not falling off the bed. I'm not sure what I was doing wrong on the first flight. I had the same seat assignment on both flights.

Also, I have come to realize that seats 5D and 5E are the seats to avoid on the 777-300ER. This row is closer to the bulkhead than the other bulkhead row in J, which means that in order to give the middle passenger enough room to get in and out of the seat, the middle seat ottoman is set off to the right side, i.e. not directly in front of the seat. So the middle passenger has a strangely shaped bed and can only get in and out via the passageway in front of 5D. And in order to make that passageway wide enough, the right corner of the footspace for 5D is "clipped," meaning the footspace tapers on the right side and scrunches the feet of the person in 5D.

I took a look at the bulkhead row in the aft section of J class and it didn't seem to have this problem, i.e. the seats were sufficiently far from the bulkhead that everyone had the same space as any other row.