Re: What is the smallest colloid?

Colloids are dispersions of one material in another.  Typically, the 
particle size of a solid in liquid colloid (sol), is less than 500 nm.  An 
important property of a colloid is the massive total surface area of the 
particles.  Colloid particles are usually too small to see with an optical 
microscope, but they can be seen with an electron microscope.  They are 
also detected by light scattering and this effect can be used to work out 
the particle size, because the intensity of scattered light is 
proportional to particle size. 

Colloids do pass through most filters so they are not filterable. You can 
get a colloid out of solution by increasing the ionic strength of the 
solution by adding electrolytes such as aluminium salts.  This causes 
coagulation or flocculation by removing the electrostatic repulsion of the 
particles and allowing them to come together and form flocs which grow in 
size until they fall out of suspension. 

I hope that this answers your questions, but I have included a very useful 
web site.

 http://www.acs.ucalgary.ca/~schramm/