A mask (N95/FFP2, not cloth) does protect the wearer. Which is why FFP2 is mandatory in several European countries in settings like public transport or supermarkets; they protect both sides. Of course if you are in a confined space with ten heavily infected individuals close by and with them at the height of being infectious, then it might not help. It is not a hundred percent protection. But if it is just one person infected and (in addition to wearing the mask) you keep some distance, then yes, it can actually help you not getting infected. Masks are one puzzle piece of trying to reduce the spread of this disease and I honestly don't understand why some people are so vehemently against them. They are a small price to pay for some return to normalcy in certain circumstances.
Also, when talking about the size of virus particles, they actually mostly travel in droplets others breathe or sneeze on you, and as long as those are larger then 0.3 microns, they won't get through, i.e. the virus won't get through. Usually the virus does not float around by itself. Someone with better knowledge please correct me if I am wrong, but as far as I know we don't breathe out pure virus, we sneeze and cough out bodily fluids that contain the virus.
Even at sizes lower than 0.3 micron an N95 / H11 grade filter will stop the virus particles just fine. Remember, 0.3 micron is the hardest to stop, smaller or bigger is actually easier in theory due to the design of material. As long as your filter is rated for efficiency.
For an overview
https://www.venfilter.com/normativa/comparative-guide-norms-classification-air-filtersG1-G4 are coarse filters, only rated for arrestance, not efficiency. Incapable of filtering particles at the submicron (1) range.
M5 onwards, the material is tested for efficiency (ie at 0.3 micron) , it will stop virus particles. N95 is equivalent to H11 on european standards.
The issue is how its used. H11 is not meant for prolonged use as part of a manual filter mask. Before the pandemic, the main market for them was for surgeons in operating theatres (which are usually already filtered with HEPA filters), 30 - 45 minutes per use, discarded after. The PPE for nurses, technicians, prolonged use are lower grade, prioritised ease of breathing for the required timescales.
Iirc boston sox red or something, who worked in PPE supply was making the same points pretty much when this issue came up at the start of this all.
In terms of disclosure, I have some background and interest on the HVAC side, even before the pandemic; the science around the transmission of Covid, not my field, same access to science journals and research as everyone else, happy to hear from anyone in the field.
In terms of giving yourself the best odds, work in space and open air if possible, if not chat with your boss to maximise the ACH (air change per hour) on your building's ventilation system. It'll up the energy costs, so don't expect this is done automatically by management.
