Display boards for science projects come in standard sizes, the most common of which is 36 inches tall and 48 inches wide. The boards are available inexpensively from office supply and discount department stores. You can also make your own if allowed by a teacher or the science fair rules, but there's no reason to get carried away with a fancy design. Your bottle rocket project should be the center of attention, not the board itself. When putting together your display, also keep in mind that you will have to transport the board and set it up, probably in a limited amount of space and time.
The rules may spell out how the information on your science board should be arranged. There is also a conventional format to follow. Generally, the board should read like the pages of a book. Place the goals you hope to achieve -- your hypothesis -- and any research or background information on the left panel. The main principles and particulars of your experiments should go on the center panel. Your results and the conclusions you've drawn from those results should be on the last panel.
Bottle rockets are simple to build and fun to fly, but there is some serious science involved. Many different principles of mathematics, physics and aerodynamics can be explored through rocketry and explained on the panels of your science board. Any good experiment starts with a question: "Do bottle rockets fly better with or without fins?" "Is there a perfect ratio of water to air for the best propulsion?" "What factors limit the altitude a water-powered rocket can achieve?" To set up your science board, ask your question, form your hypothesis, detail your experiments and explain your conclusions.
The best science projects combine solid scientific method and clear explanations, with a bit of showmanship. If the rules allow an outside demonstration, you'll have that last part covered. Since most science boards are viewed indoors, however, concentrate on some aspect of bottle rockets that can be safely but visually demonstrated, or plan on including plenty of exciting photographs.