I can do OK on the 747 BF lie-flats, 6'1" and not slight of frame....
I do always pick upper deck window. I find by lowering the arm rest and using the blanket as a bit of cushioning around the headrest/shoulders, I can sleep very well on my side.
I do not lie on my back in the lie-flat bed mode. If I want to sleep on my back, a modest degree of recline works better for me...more like a recliner than a bed.
I do sleep much better in the new seats than on the old "barcoloungers" though....