According to ASTM D4767 (Standard Test Method for Consolidated Undrained Triaxial Compression Test for Cohesive Soils) "Specimens shall be cylindrical and have a minimum diameter of 33 mm (1.3 in.). The average height-to-average diameter ratio shall be between 2 and 2.5. An individual measurement of height or diameter shall not vary from average by more than 5 %. The largest particle size shall be smaller than 1/6 the specimen diameter. If, after completion of a test, it is found based on visual observation that oversize particles are present, indicate this information in the report of test data"