Blades typically come in 10 or 12-inch sizes. Deciphering between which saw to purchase is dependent on your use cases. However, unless you have a frequent need for a 12-inch blade a safe bet would be to buy a 10-inch.
The 10-inch option: At most hardware stores there will be a larger selection of 10-inch blades that you will also find to be less costly. Not only is the upfront costs less, but fewer teeth means these blades are also less expensive to sharpen. The smaller radius allows the blades to spin faster using fewer amps ultimately driving down the cost of electricity, too. A good 10 inch option is the DW717 made by Dewalt.
The 12-inch option: The benefits of the 10-inch saw do not always outweigh that of the 12-inch. Bigger blades mean longer cuts. These tools also use more amps so they drive more power into the cuts. This is ideal for tasks like deck building or projects that use thicker material. Higher tooth speed also means cleaner, sharper cuts. A 12inch saw I really like is the DWS780 from Dewalt.
Purchasing a laser for your miter saw would be a good idea if you if a project requires accurate cuts. Laser essentially act as a guide by shining a beam onto the material where the blade will hit upon lowering. This way, a woodworker will know precisely where the blade will cut before contact is made. This decreases the risk of error and is a safer way to use the tool.
Keep in mind the laser is harder to see under bright light, so this is best for indoor use.
Again, when determining which type of miter saw to buy you need to consider what you will be using it for.
If you are creating simple things that require angled cuts in two planes (like picture frames, or crown molding) you will want a compound saw. These miter saws can pivot left to right and can create a compound cut in one easy pass.
Sliding miter saws differ from compound saws because they can also move forwards and backwards. This allows woodworkers to increase the length of a cut. They can do all that a compound saw can do and can cut up to 16-inches thick. This is ideal for lumber, boards, or logs.
So, now if you know what type and size miter saw to buy, the last thing you need to know is which brand to choose. A quality woodworker needs a reputable tool!