How do salt additives and impurities affect a critical micelle concentration?
In a solution with two dissociated ionic detergent molecules of equal charge (say two anionic detergent molecules $\ce{A^-}$), their interaction will be dominated by two effects: 


Electrostatic headgroup repulsion 
Hydrophobic interactions


The first contribution, electrostatic repulsion, will oppose the second and discourage association of the detergent molecules.

If you instead mix two oppositely charged detergents, $\ce{A^-}$ and $\ce{B^+}$, the electrostatic headgroup interaction will now encourage association. All else (e.g. tail length) being equal, this means mixed micelles will have a lower critical micelle concentration (CMC) than pure detergent at the same total concentration. 

Adding salt (say $\ce{NaCl}$) to a charged detergent will also lower the CMC, for essentially the same reason as for mixed micelles (electrostatic repulsion is reduced).

Now comes the tricky part. If you add salt to a mixed detergent system, you set up possible competition between electrostatic attraction of salt and headgroups, with electrostatic attraction of oppositely charged headgroups. Individually, either attractive interaction has a tendency to lower the CMC. The question is what effect they will have together. My educated guess is that the effect of salt on mixed micelle systems will be subtle and complicated. The CMC will be further reduced, but interactions between oppositely charged detergent molecules will predominate so the effect of added salt will be minor. 


  When you lower a CMC, are the micelles themselves now smaller or bigger? And when the literature describes micelles as smaller, does this mean the circumference is smaller, or the number of detergent molecules is smaller?


I don't think there is a universal answer to the first question.  I will have to wave my hands a little, and say it depends on the detergent, solvent, and temperature. However if you look at a table of aggregation numbers and CMCs for different detergents, you'll usually see that the ones with a lower CMC (for instance, nonionic detergents) also have larger aggregation numbers. 

The second question is easier to answer than the first.  The size of a micelle can be measured by a number of different techniques. The aggregation number is often derived by making assumptions about packing in the micelle and starts from estimates of the sizes of the micelle and detergent molecule. So it's pretty safe to generalize that both the dimensions of the micelle (the effective diameter say) and aggregation number are reduced in smaller micelles.