A cubical triaxial tester (CTT) was developed and has been used successfully for determining fundamental mechanical properties of powder materials by the Pennsylvania State University's Powder Mechanics group. The CTT uses a cubical test specimen with the dimension of 50.8 × 50.8 × 50.8 mm to allow the measurement of large deformation. An adapter for the CTT to use a smaller cubical sample with the dimension of 31.8 × 31.8 × 31.8 mm was developed to accommodate needs for the smaller sample volume. To examine the ability of mechanical properties determination with the new small sample, a set of hydrostatic triaxial compression tests and conventional triaxial compression tests were performed. Most elastic and elastoplastic properties are not significantly different (P > 0.05) while plastic parameters measured using large and small size samples had lower P values. Failure stress differences of large and small size sample showed even lower P values. Only failure stress differences with 5 kPa of confining pressure showed significant difference (P < 0.05) between large vs. small test sample.