"size of type of about six lines to the inch" (12 point), 1580s, probably from pica, name of a book of rules in Church of England for determining holy days (late 15c. in Anglo-Latin), probably from Latin pica "magpie" (see pie (n.2)); the book so called perhaps from the color and the "pied" look of the old type on close-printed pages. The type size was that generally used to print ordinals.
"pathological craving for substance unfit for food" (such as chalk), 1560s, from Medieval Latin pica "magpie" (see pie (n.2)), probably translating Greek kissa, kitta "magpie, jay," also "false appetite." The connecting notion may be the birds' indiscriminate feeding.