Expanded polystyrene (EPS) geofoam has now been used around the world as a remarkable lightweight soil substitute for a variety of geotechnical applications in soft ground construction. Property characterizations of EPS for determination of design parameters have relied on laboratory testing of small samples. For the most part much of what is commonly known is based on data derived from unconfined compression testing of standard 50 mm cube samples. Performance observations suggest that Young's modulus values derived from laboratory tests when used in analysis tend to overestimate deformations. Available information on Poisson's ratio values widely varies. Little is known about the influence of sample size on desired elastic parameters. Studies of EPS behavior under unconfined compression were conducted on both cubic and cylindrical samples of different material densities. Both axial and lateral deformations were monitored to enable simultaneous evaluation of Young's modulus and Poisson's ratio. The results indicate significant sample size influence on elastic parameters. Young's modulus and Poisson's ratio values tend to be underestimated. The cause for the under estimation is attributed to be due to non uniformity and end effects in the proximity of the rigid loading platens.