Careful, Izzy. I think that this is one of those situations where a little knowledge taken in isolation can lead to an incorrect conclusion. Jeff already covered it, but basically, people sick with the flu don't sling bare, individual virus particles around when they cough or sneeze, so the size of one virus particle is not that important.

Influenza generally is transmitted through "large" droplets when someone coughs or sneezes, and those are generally in the 10 micron range. Large enough to be trapped by the 0.1-0.3 pore size of your average N95 respirator. Smaller, aerosolized particles are also possible, but the predominant transmission method is thought to be through these droplets. And even aerosolized particles will likely be larger than the average pore size of the filter.

Unfortunately, trying to protect yourself involves more than just sticking an N95 on your face. I've said it in the past, but without the knowledge and training to know how to put it on and make sure it fits properly, know what situations to use it in, know when it's safe to take off, know how to take it off so you don't accidentally infect yourself, know when to change a mask, etc., having an N95 could just provide a false sense of security. A person might put themselves at greater risk, like being among a crowd of people, because they think the mask protects them when in fact, it really isn't because, for example, it doesn't fit properly.

There is still little or no good evidence that masks provide any significant protection to the general public (as opposed to trained healthcare workers). For average folks, strategies like hand washing, cough etiquette, avoiding crowds, etc. are going to be more effective overall in helping control any outbreak in your community since they go the heart of disrupting the habits that get us infected. And knowledge is not something that you can run out of, like you can with masks.

Jeff mentioned another important point--masks can help keep sick people from infecting others. In Mexico City, where soldiers are giving various types of masks to people, the logic may be to minimize the risk of infected people from spreading the infection, rather than trying to protect the healthy from getting infected. Or it could be more of a psychological benefit to receive a mask, but to me, I think the comfort of having a mask is outweighed by the feeling of seeing everyone else around you with a mask on.