Marine organisms are often classified by how they move (e.g., plankton cannot swim against a current), where they get their energy (e.g., "phyto" usually applies to photosynthetic organisms), and their size (see chart below). This overview is intended help "sort out" the confusion!
These "generic" names classify plankton by size
This shows typical size ranges for some common marine groups
This shows the range of sizes for these classifications

Megaplankton are the largest plankton. They range in size from 20 centimeters (8 inches; about the width of a sheet of paper) to 200 centimeters (6.5 feet; around Michael Jordan's height). Floating animals in this size range are called "megazooplankton."
Macroplankton are relatively large plankton. They range in size from 2 centimeters (0.8 inches; about the width of a U.S. nickel) to 20 centimeters (8 inches). Floating animals in this size range are called "macrozooplankton." Floating plants in this size range are called "macrophytoplankton."
Mesoplankton are from 200 micrometers (about 4 times the width of an average human hair) to 2 centimeters (0.8 inches). Floating animals in this size range are called "mesozooplankton." Floating plants in this size range are called "mesophytoplankton."

Microplankton are between 20 micrometers and 200 micrometers in size: i.e., from about half a hair width to about four hairs wide. Floating animals in this size range are called "microzooplankton." Floating plants in the this range are called "microphytoplankton."
Nanoplankton are between 2 and 20 micrometers (i.e., much smaller than a hair's width). The smallest phytoplankton are in this size range (see chart at top). Special techniques are often used to identify these tiny organisms; e.g., staining the cells' cytoplasm and viewing them under fluorescent light.
Picoplankton are between 0.2 and 2 micrometers and are commonly called "marine bacteria." Members of the picoplankton include Prochlorococcus and Synechococcus, species responsible for much of the photosynthesis in the world’s oceans. Like nanoplankton, special techniques are often used to identify bacteria; e.g., staining the cells' DNA and viewing them under ultraviolet light (>>).
Femtoplankton are less than 0.2 micrometers and are commonly called "marine viruses." Viruses are neither cells nor considered to be "alive": they consist of a protein coat around a clump of genetic material.