A question often asked by prospective sliding miter saw buyers is whether to go for one with a ten inch blade, which will be cheaper, or whether to go all out for a twelve inch machine. In this article you can find out the pros and cons of each.
The first point to make is that we're talking about saw blade diameter when we say "12-inch" or "10-inch" etc. With a non-sliding miter saw, there is a direct relationship between the blade size and the maximum cut size, but with a sliding miter saw it's not quite the same because of course the slider extends the cut length. However, even taking that into account, a twelve inch saw is going to be able to make longer cuts because, basically, everything tends to be built bigger.
Compound Saw
Indeed, it's not just the length of cut that's bigger with a twelve inch saw. The depth is also greater because the machine designers have that extra blade surface area to play with. For some people this extra depth capacity alone makes the twelve inch a necessity.
On the other hand, many people find that a ten inch sliding miter saw is big enough for 95% of jobs around the home, including all the obvious things like decking and crown molding. This means it can be hard to justify the extra expense of a twelve inch sliding miter saw; and we're talking maybe an extra 0 or 0 in cost; just to cope with the 5% of projects that the ten inch saw can't handle.
Something else to mention is that if you think the ten inch saw won't cope with bigger cross-cuts, remember that a table saw rather than a sliding miter saw is really the tool of choice for big cross-cuts. So, if that's the main reason you would need a twelve inch miter saw, I would get a ten inch saw and then think about buying a table saw to go with it. This combination will give you a great deal of flexibility and you will be able to use exactly the right tool for the job in hand.
Let's face it, however big a machine you get, there is always going to be some job sooner or later that it will not cope with, so for most people a compromise that handles 95% of the work is about right. The problem is that this 95% will comprise different things for different people. If you regularly make wide shelves, or if you need to cut, say, six-inch crown molding on a regular basis, then a ten inch sliding miter saw is just not going to work for you.
Personally, I try to predict what work I will need to do over the next year or so and buy a saw that will cope with the 95% benchmark. Anything more and the price cannot really be justified.