During unconfined compressive strength (UCS) tests of oil well cement, it is not uncommon for standard cubic test specimens to require some processing in order to meet the requirements of end face planarity and parallelism. Also, unique research scenarios may produce cement specimens for which UCS data is required but that deviate slightly from the 50.8 mm cube shape recommended by API for testing UCS of oil well cements. Little has been published with regards to the effect of such minor specimen shape variations on the UCS value measured or on the acceptability of the obtained measurement. Thus, different oil well cement formulations of varying compressive strength were tested for UCS using different specimen shapes and sizes in order to quantify the effect of geometry on the measured UCS. The shapes and sizes tested were chosen to cover the range of sizes that could result due to slight deviations from the API-recommended standard testing geometry. The validity of the UCS measurement for each specimen geometry was also tested for validation using statistical techniques outlined in ASTM C109 and ASTM C670. The results of this study show that there are no discernable direct size effects for the range of sizes tested even though the shape and size strongly affect the reliability or validity of the UCS reading.