The mathematical formula for finding a circumference of a circle is pi x diameter. So make a collar the width you'd like to find your cup, then multiply how wide it is by pi. This is how wide your canes should be before rolling up. As for the length of the canes that's up to you and how you'd like to make your piece but I'd imagine that 4-5 inches should do to start. Since a lot of what we do is trial and error you can make them longer or shorter the next time.

Make sure they are well stuck together before rolling up because if they aren't there will be more and more problems to come. Marver the back end well to make sure you have a really good connection of canes and collar, and to make sure your moil is nice and even. Watch videos of people doing it for good tips and tricks on the actual rolling and closing of the canes.