I'm an ex asbestos inspector and management planner for schools and OSHA certified on respirators. First off, do you have a beard? If you do you need a full face respirator which are a pain to wear and even they don't work if your beard is too long. That said get a half face respirator, preferably from a place you can go to to be fitted, like an industrial supplier. If you can't be fitted you'll have to guess for a small, medium or large size respirator. Tighten it onto your face and get a violet/purple HEPA filter. HEPA stands for high efficiency particulate absolute filter. It blocks particles smaller than most bacteria, or 0.3 microns. Also as a tip I use to tape a layer of paper towel over the ends of the HEPA filter to keep them from getting blocked up too soon. You'll know when it's getting clogged up. I'd remove the paper towel and use the blower end of my shop vac to blow it out. Make sure the vac is clean and blowed out first! Not supposed to do that according to regulations, but it works for some time. An air compressor works too, but don't hold it too close or it will spread the fibers of the filter apart.
The best way to test if it fits you is to use ammonia fainting ampules, or smelling salts and have someone use it around the outside of where the respirator contacts your face. If the mask is the wrong size or not on tight enough you'll get a big dose of smelling salts, aweful. If you don't have smelling salts try a stinky cigar, it blocks the particulates in most smoke that carries the smell. Be aware it is not a fume filter, so if using acetone you'll need a NIOSH approved fume filter, I don't remember the numbers. And gases like carbon monoxide from the smoke is you will need a fresh air supply. Fume or particulate filters don't block gasses.
By the way if you had to buy a mask and it doesn't fit, you cannot return it to a an online company in most cases. So if ordering from some company like MSG be sure and make a good guess. Here's a link to help figure out and remember, if you're breathing dust in small amounts you may not feel it until your coughing up crap. So check and make certain it fits, do the stinky test!
https://www.unr.edu/Documents/resear...structions.pdf
http://https://www.envirosafetyprodu...-the-size.html