Did Melville indicate how big Moby Dick was?

That’s not a painting; it’s digital artwork by Chih-Han Hsu. I’m not sure about the whale itself but the figures in the foreground look to be 3D models.

presumably at least as long as a Kentuckian is tall

Chapter 95 - The Cassock

http://img855.imageshack.us/img855/9538/5d9d.jpg
http://fc08.deviantart.net/fs71/f/2010/295/c/e/megafauna_size_comparison_mk2_by_harry_the_fox-d2xt9nb.jpg

Sometimes a whale is just a whale.

IIRC that it turned out that sailor had actually spotted a giant squid, not the White Whale. Again, IIRC the squid is neither aggressive nor afraid of humans and everybody is a little freaked out.