I think the distinction of "smallest plate" get's a little fuzzy. Right off the top of my head there's the Juan de Fuca and Gorda plates off the Pacific Northwest which are pretty small.

Taking a jaunt over to wikipedia, there they are listed in groups as primary, secondary, and tertiary plates.

Primary of course are the seven we all know and love.

The secondary lists the Arabian, Caribbean, Cocos, Juan de Fuca, Nazca, Philippine Sea, and Scotia plates.

The tertiary list has about 60, including the previously mentioned Gorda, as well as the Okhotsk which is a microplate of North America way over behind Japan that the Pacific plate is subducting under, which was involved in the recent Tohoku quake.

Off of Central and South America per your question are the Cocos and Nazca secondary plates, and of course the South American primary plate. Associated with these plates are the following tertiary plates...

Cocos:

Rivera Plate - "located off the west coast of Mexico, just south of the Baja California Peninsula. It is bounded on the northwest by the East Pacific Rise, on the southwest by the Rivera Transform Fault, on the southeast by a deformation zone, and on the northeast by the Middle America Trench and another deformation zone."

South American:

Altiplano Plate - "located in southern Peru, western Bolivia and far northern Chile. It consists mostly of the central Andes and the Altiplano of Peru and Bolivia. The western border is where the Nazca Plate subducts underneath this plate."

North Andes Plate - "located in the northern Andes. It is squeezed between the faster moving South American Plate and the Nazca Plate. Due to the subduction of the Nazca Plate this area is very prone to volcanic and seismic activity."

Info from wikipedia.

http://en.wikipedia.org/wiki/List_of_tectonic_plates
http://en.wikipedia.org/wiki/Altiplano_Plate
http://en.wikipedia.org/wiki/North_Andes_Plate
http://en.wikipedia.org/wiki/Rivera_Plate

Hope this helps.

Brian