Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
circĕs, itis, m. [circum-ire], a circle (ante- and post-class.): ut parvi circuli Anuli, sic magni dicebantur circites Ani, Varr L. L. 6, § 8 Müll.; Sid. Carm. 22, 222.
So of the circumference of the circus, Sid. Carm. 23, 381.