Well, you can pick a PC skin style (in the lower left corner of the whole screen) that's very wide or fluid width even, and make sure to press the "Go Advanced" button before posting, and then "Preview" your post so you'll make sure that it's at least 4 lines long.
If you find you're having a hard time filling up a post, think about circumstances, feelings, surroundings or gestures that you can describe to make the post feel more alive.