There are 2 questions to answer to decide on the correct blade:-

1. What material are you going to cut?

There are various diamond tips for materials of various hardness and abrasiveness.

You should choose a tip type to suit the material that you will be cutting. This will give the best blade life and productivity.

If you are cutting reinforced concrete, we recommend that you use a special concrete cutting blade. It has smaller slots so that the reinforcing bars do not knock the tips off the blade.

2. What is the maximum depth of cut you need?

It is most economical to use the smallest blade suitable for your saw. Why?

Diamond blades are made from expensive, high quality steel. The larger the diameter of the blade, the larger the area of the disk, and the thicker the blade must be. The volume of steel used increases at a much higher rate than the increase of cutting depth.

And you will be paying for it

Smaller blades will also give higher productivity as they are thinner than large blades and remove much less material while cutting.

Regardless of how deep you need to cut, don't choose a blade that is too large for the power of your saw. It will cause the blade to stall as the motor will not have enough torque.

Saying all of this, a too small blade will not have high enough tip-speed to cut effectively.