I mean, I wasn't super active in fandom then, so I can't speak with any real authority about how people thought of fic length. I know that I definitely think of fics in terms of fractions of an LJ post, but I could be special that way. You're definitely right that people started writing way more shortfic, which might have contributed to a change in perception about longfic. The real biggest difference in my mind is the shift away from serial fic - stories that were intended to be read as WIPs and possibly continue on forever - to oneshots (which also contributes to the feeling of fic getting shorter, since the serial fics that would get emailed out sometimes seemed infinite in length).