Is 50,000 the magic number for e-books?

I was talking with a friend this afternoon who is something of an industry watcher in the publishing industry, and we were discussing e-book length. Now I've thought about the length of e-books for a while, so it was a familiar discussion, but he brought some interesting ideas to the table.

His take on things is that thinking of e-books in terms of the conventional publishing market is wrong, because they simply aren't paper books and don't follow the same psychological rules as books. Size...