What size is best for someone’s first attempt at making a telescope mirror?
Most books on telescope making recommend starting with either a 6- or an 8-inch mirror, and that’s good advice. Either size is suitable for a first-timer, but a 6-inch will probably take a little less time to complete.
What is perhaps an equally important consideration is the mirror’s focal ratio (focal length divided by diameter). In general, long-focus mirrors (f/8 or greater) are easier to make but harder to test, whereas short-focus mirrors (f/5 or less) are harder to make but easier to test. For this reason I usually recommend f/6, which provides an excellent balance between the demands of testing and fabricating.
But what if you really want a big mirror? Most experienced glass pushers say that it will take you less total time to make a 6-or 8-inch first and then a big one, as opposed to starting right off with a big mirror. The basics of mirror making are pretty much the same regardless of size; only the amount of time required for each step changes. But the advantage of a smaller mirror is that you can more quickly undo a mistake and get back on track than you can with a big mirror — and with a first mirror, you can count on making plenty of mistakes!
— Gary Seronik