When I worked in radiation and other hazardous areas we had to be fit tested before we could be issued a respirator, as some have already pointed out. With some types there were different sizes, and some people couldn't be fit to any size, and so could not be allowed into areas with airborne radioactivity; I don't remember full face respirators having different sizes, but I do remember some people not passing the fit test with them (usually men with big heads). Also, we had to be clean shaven, except for a mustache, as has been pointed out. And you had to shave every day, because even a little stubble could break the seal. You can do a field test on your mask's seal by removing the filter and covering the opening with your hand and trying to breathe in; if you can't breathe in at all and the mask pulls toward your face, your seal is probably good. But don't do that test half-assed, or you might screw yourself.
If there is airflow to the mask, as when we had airlines running to us, a good seal is probably less important, because there is a positive airflow out of the mask, and you won't be sucking in outside air.
Remember that a mask is only one part of avoiding contamination. There are very strict procedures that have to be followed too. I have no experience with bio-hazards, but I know that with radioactive contaminants, any part of your body or protective clothing might be contaminated. So your mask might work perfectly at keeping you from breathing in airborne radioactivity, but later after you take it off something that got on your clothing finds its way inside. Doing it right is pretty complicated, and even the professional health physics people in the places I worked didn't always get it right. And I'd guess that biological contaminants are probably at least as problematic as radioactive ones.