Paper ballot operational complexity scales linearly with the size of the electorate, which makes them adequately scalable for any practical use. (There's maybe an issue with using paper ballots for some esoteric election methods, because of how operational complexity scales with number of candidates for some type of tallying, but absent a decision that use of one of those methods is desirable that's immaterial.)

Australia holds elections this way and has done so for a century. Scaling has not been an issue. Neither has transparency.