Anisocytosis describes the variety of sizes of erythrocytes (e.g. simultaneous occurrence of microcytes and macrocytes in the peripheral film).
Typically occurs in recovery from microcytic anemia, i.e. with the release of young erythrocytes which are larger. An excellent example is iron deficiency anemia after iron therapy. This often occurs in other forms of anemia as well.
A mild anisocytosis is a frequent and unspecific variation of the red blood cells.