I agree about aging Alan down--even when I watch the films, in my head, he's thinner, a little less jowly. :P I think the trouble with Dan is that a lot of people also like Harry short for a tall Snape and by DH Harry is taller (but since DH is not exactly most people's favorite book :P it goes unnoticed/forgotten).
I am terrible with remembering art and fic--I've read and seen it all but it's muddled in my head. O_o