The term 'microcytosis' means small red cells. The three main causes are iron deficiency, some cases of the anemia of chronic disease, and the thalassemia syndromes.
Sideroblastic anemia is one of the rarer causes.
The illustration here is from a patient with iron deficiency. It is hard to judge red cell size from a blood film, but as a rough guide, the red cell should be about the same size as the nucleus of the small lymphocyte.