A rain gauge can have any dimensions, but it is better to use a tall thin cylinder with straight sides and flat bottom so that the amount collected can be seen more clearly. The best thing for you to do is to magnify it by calibrating it. First, get a funnel. Next, get a beaker of the same Ø as the mouth of the funnel. Fill this beaker up with several inches of water and then pour it into you tall narrow cylinder. Divide the level of water into equal spaces. Next drain away the water and place your funnel in its mouth. Finally, place your cylinder outdoors in an open area where it can collect rain.
I hope this will help you with your little science project.
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If only you knew the power of the Dark Side of the Force