I guess it depends what you consider small, but the Ocean (Formerly the Tahitian Princess, renamed in 2009) and Pacific are the smallest of the Princess fleet. They only carry 680 passengers.
Most of the other ships are in the 2000-3000 passenger range.
After the Ocean/Pacific the next smallest would be the Sea which carries 1950, then the Island/Coral which is 1970 passengers then the Sun/Dawn which carries 1990.
After that is the Grand class which are all 2600 or more.