Poll

It actually depends on how fast I need to finish the book. 320 pages can seem really long when you have 20 books looking at you. But really, the longness of books isn't defined by the amount of its pages. At least, that's what I think. Charles Dickens has 900+ paged books, and they don't seem all that long to me. But there's some books out there that are 200+ books, and they are excrutiatingly long because I dislike them so much.

I consider a book long if the plot goes really slowly. If I'm reading a good, fast-paced book, +700 pages would be too short for me.

Also, it depends on how much I like the book. If I am not enjoying the book, but I am forced to finished, like say for school, then it will seem very long even if it is only a couple hundred pages.

I've noticed quite often that books over 500 pages tend to drag a bit. They tend to be overloaded with description and confusing plot points that, most often, seem to have been put there specifically to lengthen the book. Of course, most books have lots of padding, which isn't always a bad thing. But when you get to over 500 pages of said padding... it gets old.
Of course, some of the "longest" books I've read, were actually only 300 or 400 pages. They just... never really got off the ground.
I don't mind reading longer books, in fact I've been known to enjoy it. But there's something that happens to a story when it crosses that 500 page mark, and it's not usually a good thing.
Sort of a like a movie going over 3 hours. There are very few films which can justify having such length (offhand, I can think of two).

depends on the font size

I read books exceeding 1000 pages and didn't feel their length, whereas I read others having less than 200 pages and felt they lasted an eternity, so I won't be able to reply to this. The length is determined by the contents and the style of the author. Some books make me want to stay inside them longer and appreciate every word like others savor the rarest of delicacies :)