The most used particle size turns around 50 µm. In no case it should be less than 20 µm because the porosity of the PE frits that retain the solid phase is about 20 µm. Regards.

Thank you for your help, I really appreciate it.

if You are going for (offline) SPE I do agree to Elias. You could also imagine running a real separation column (online) than You would rather work with 4 µm particles.