Length is based solely on the content of the movie, so if one film is 90 minutes long, I can like it just as long as I like another film that's 3 hours long. The problem is when people try to tell 90 minute long stories in a 3 hour movie, or vice versa.
EDIT: By the way, I think Serenity has claimed my top spot for best movie of 2005... even though I didn't see it until 2006.