He usually makes bigger circles in the lighter areas of the subject and smaller circles in the darker places. This gives more volume and a 3D illusion. There is no limit, it’s a new technique and he think's there is much more to do with it.
Mad Hatter - Johnny Depp
This is the "circlist" portrait of Johnny Depp as seen in Tim Burton's movie "Alice in Wonderland". It took several days of work to make it... He digitally applied each colored circle on a black background.
The original version of this image is huge (11 000 pixels wide, 300 dpi).
Lady Gaga
The original version of the above portrait is huge (10000 pixels wide).