CC-CEDICT: 坐井观天 (zuòjǐngguāntiān) lit. to view the sky from the bottom of a well (idiom); ignorant and narrow-minded
The origin of the above 成语 (idiom) is reported to be the line
坐井而观天 to sit in a well and observe the sky
曰天小者 and declare 曰 the sky is a small thing 小者
非天小也 [but it is] not 非 the sky which is small
The 也 at the end seems to be the special character:
CC-CEDICT: 也 (yě) (in Classical Chinese) final particle implying affirmation
Honestly, my understanding of this sentence is fairly flimsy, especially 曰, 小者, and 非.
Question: How do I precisely understand 坐井而观天，曰天小者，非天小也?