Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
per-magnus, a, um, adj., very great, very large: accessio, Cic. Fin. 1, 17, 15: vis naturae, id. ib. 2, 7, 18: hereditas, id. Verr. 2, 1, 10, § 27: numerus, Caes. B. G. 7, 31.
As subst.: permagnum, i, n., a very great thing, something very great: permagnum existimans tris Olympionicas unā e domo prodire, Cic. Tusc. 1, 46, 111: permagni interest, id. Part. 24, 84: permagni refert, Ter. Heaut. 3, 1, 58; so in tmesi: per enim magni aestimo tibi factum nostrum probari, Cic. Att. 10, 1, 1: decumas permagno vendere, at a very high price, id. Verr. 2, 3, 39, § 90.