mid-15c., "capable of being considered, conceivable," from Medieval Latin considerabilis "worthy to be considered," from Latin considerare "to look at closely, observe," probably literally "to observe the stars," from assimilated form of com "with, together" (see con-) + sidus (genitive sideris) "heavenly body, star, constellation" (see sidereal).
Meaning "pretty large" is from 1650s (implied in considerably), from now-archaic earlier sense of "Worthy of regard or attention" (1610s).
CONSIDERABLE. This word is still frequently used in the manner pointed out by Dr. Witherspoon in the following remark: "He is considerable of a surveyor; considerable of it may be found in the country. This manner of speaking prevails in the northern parts." [Pickering, "A Vocabulary, or Collection of Words and Phrases Which Have Been Supposed to be Peculiar to the United States of America," 1816]