Is it possible to go beyond 20480m? Probably. I don't know if I'm willing to find out though... at this point, even in fullscreen, I can barely see the stack at all (which is approximately 80 blocks wide at its thinnest). I'm not sure how I even placed the last block correctly to get past 10240m.
In theory, I used twice as many blocks than would be strictly necessary, so maybe the next level is possible. You would have to make a much thinner tower than I did though, which would mean it'd probably be actually invisible at this point, making placing blocks accurately pretty much impossible
