Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
dī-mētĭor, mensus, 4, v. dep. a., to measure any thing, to measure out (v. demetior init.—rare but class.): studium dimetiendi caeli atque terrae, Cic. de Sen. 14, 49: dimetiri et dinumerare syllabas, id. Or. 43, 147: campum ad certamen, Verg. A. 12, 117: mundum, Quint. 12, 11, 10; cf. id. 9, 4, 112.
Hence, part. as subst.: dīmētĭens, entis, f., the diameter (διάμετρος), Plin. 2, 23, 21, § 86 sq.
Note: In pass. signif.: columnae altitudo dimetiatur in partes duodecim, Vitr. 3, 3; 5, 9; Quint. 8 prooem. § 27: mirari se sollertiam ejus, a quo essent illa dimensa atque descripta, Cic. de Sen. 17, 59; so in the part., Caes. B. G. 2, 19, 5; 4, 17, 3; Quint. 9, 4, 27; Cic. N. D. 2, 62, 155; Verg. G. 1, 231; 2, 284.
Cf. in fut. pass. part.: non cum vitae tempore esse dimetiendam commemorationem nominis nostri, Cic. Arch. 11, 29 Halm (Baiter, dimittendam).