Expanded polystyrene (EPS) is a sustainable material that has high potential in civil engineering, especially in roadways or railways, due to its properties, such as ultralight weight, high strength to density ratio, ease of construction, and inert nature. EPS blocks can be used in civil engineering as embankment fill material, inclusion in retaining structures, vibration dampers, etc. In the field, EPS blocks of different sizes are used that are subjected to different loading conditions during and after construction. In the present study, the effect of several parameters, such as EPS block size, loading conditions, and precompression on the EPS stress–strain characteristics, is investigated in the laboratory. The effect of EPS size is found insignificant but the loading condition affects the EPS response considerably. Numerical modeling is used by previous studies to explore the use of EPS in different applications. The selection of suitable constitutive model and the determination of input parameters affect the outcome of the numerical studies significantly. Hence, in the present study, a discussion on the selection of the constitutive model along with the determination of elastic and strength parameters for EPS through laboratory tests, such as unconfined compression tests, triaxial tests, and direct shear tests, is covered. The selected numerical model for EPS is validated with laboratory tests, and the limitations of the validated model under practical situations are explored.