Reading this article in conjunction with Berkey Micron Rating will satisfy this question as answered.

These organisms are not bacteria or viruses, but protozoans with complex life cycles. Outside of the host, the organisms are in a cyst stage, which is much like a seed for a plant or microscopic egg.  The size of the Giardia cyst is approximately 8 to 14 um (microns); whereas, the Crytosporidium oocysts are usually 4 to 6 microns in diameter.  (Note: 1 micron = 0.001 mm = 0.00004 inches ), which is too small to see with the naked eye. These organisms are reasonable for over 100,000 causes of reported outbreaks of waterborne, since 1979.  These are not new organisms and in fact Van Leeuwenhoek first describe Giardia cysts in 1681, but these organisms have not been identified as disease causing agents until the last two decades.

Resource; Water Research Centre 

US EPA minimum filtration requirements for surface water to remove Giardia and Cryptosporidium

Black Berkey® Purification Elements exceed these requirements at Log 5 (99.999%)  

At least 99.9% (3-log) removal and/or inactivation of Giardia lamblia cysts

AND

At least 99.99% (4-log) removal and/or inactivation of viruses. The above treatment objectives can be met by combining filtration and inactivation (disinfection) barriers. You can add up the logs achieved from each barrier to get the total treatment for that micro-organism

AND

At least 99% (2-log) removal of Cryptosporidium

Note that for Cryptosporidium, the IESWTR and LT1ESWTR require that 2-log removal be achieved by the filters; inactivation of Cryptosporidium is not credited for compliance with these two rules. Additional removal and/or inactivation of Cryptosporidium based on LT2ESWTR monitoring results or commitment to install maximum treatment may apply. Exact level of additional treatment required is system specific

Refer: US EPA