Traditional comic books have a set length determined by the printer (usually 32 or 64 pages). If you don't intend to print your book in the traditional manner, you can make it as long or as short as you like, but the principles of composition still apply. Plan each comic book either as a complete story or a complete chapter--evenly paced with a coherent beginning, middle and end. If you plan to end your comic book on a cliffhanger (as many big-league comics do), time it so that it ends on the page you need, and be prepared to adjust your script if it doesn't fit the parameters of length.
Comic-book writers compose scripts for two specific people: their readers and the artists who will draw, ink and color it. The artists require a certain amount of direction, and you'll need to tailor your script to match their expectations. Some want just a bare-bones description of each panel, covering the basics and then filling in the details themselves. Others require detailed descriptions of what is expected, down to the last millimeter of space. Most fall somewhere in between, but as you work with each artist, attune yourself to his needs and learn to balance them with yours. At the same time, don't become so engrossed in your relationship with the artist that you neglect the reader. Make sure the plot can be easily followed from one page to the next and that your audience never grows confused by the action.
Both individual pages and individual panels require a careful approach from the comic book writer. You can place only a certain number of panels on each page (eight is normally the limit) and each panel has to convey enough action to differentiate itself from the next. Moreover, the dialogue you write needs to fit comfortably within each panel, leaving enough room for characters and action to appear alongside of it. (That's one of the reasons why superhero catchphrases developed--they're a good way of conveying character without taking up a lot of page space.) Keep your dialogue as sparse as possible and gauge how much you use per panel. Your finished comic will benefit in the long run.