You can just write what you feel - something might come to you in line, you never know~ :) You can write as much or as little as you want to as well.
(If people are only writing short amounts though, I'd prefer they don't take up the WHOLE page for just saying, "I LOVE YOU!" - I want to make sure there is room for everyone who wants to put something in to do so, but at the same time, I want the book filled. XD;; Ahhh, such a dilemma. xP)

And you'll have time in line too, don't worry. If you wanted to draw or design something creative, you can take as long as you need, don't worry. We have all day, after all. ;)