Generally, medium will be suitable for the average size women and large for the average size man. Thats what the website says, I went along for the large and I'd agree.

I commute to work through the centre of London (15km per day) and decided the best mask was the Sportsta with anti-pollution filters. The Sportsta has vents at the side so you don't suffocate when you're in need of air (though when it's very wet, this function fails and you have to remove to breathe - though this rarely happens). I started with normal filters, but looking at the manufacturer website, this was not specifically designed for urban cycling. The anti-pollution filters have activated carbon so pollutants and particles adsorb to this instead of going into your lungs. As an asthmatic, I've found this combination fundamental. According to the site I change the filter once a month, and you notice the dark marks where the pollutants have been trapped by the filter.