Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
immensĭtas (inm-), ātis, f. [immensus], immeasurableness, immensity (Ciceron.): latitudinum, longitudinum, Cic. N. D. 1, 20, 54: immensitates camporum, id. ib. 2, 39, 98.