Here you choose the correct diameter specified for your machine. The only exception would be if you need to reduce your blade speed (also referred to as surface feet per minute, or SFM) on a fixed RPM machine.

If you have a fixed speed machine and need to slow the SFM down, AND you will be cutting small enough stock, you can put a smaller blade on the saw than what is called for.

Example: If you have a machine that takes a 350mm blade and runs at 27 RPM, your blade will be cutting at 97 surface feet per minute. If you can cut your through your material with a 275mm blade, your SFM will be reduced to 77 SFM.

This may increase your tool life significantly in some applications (tough/hard materials like Alloyed Steels or some Stainless Steels).