How to calculate the maximum size of a particle where its gravitational force no longer dominates?

I read in a paper that as particle size decreases and it reaches a limit, the particle no longer has gravitational force as its dominant force and it starts to travel in the air for long times. Aerosols are such particles.

1. Which force is dominant on such particles then?
2. With that knowledge, is it possible to find the largest particle size that starts to float and doesn’t settle down?