I agree with Mike, start with a 6" or even an 8". Both are a good beginner size. Also if you make the mirror an f/8 or so, it will be a lot easier to figure than a faster f ratio of lets say a f/4 or f/5. Plus it will have more power for planets. Its when you take on a large mirror that the battle begins. The 20" f/5 I am currently working on is a real undertaking. The weight alone is enough to kill the back.