The effects of size and aspect ratio (height/diameter ratio) of the specimen on the mechanical properties of a Zr56Al10.9Ni4.6Cu27.8Nb0.7 metallic glass have been investigated by uniaxial compression tests and scanning electron microscope (SEM) observations. The results show that the metallic glass with smaller size and lower aspect ratio shows an enhanced plasticity due to larger amount of free volume formed during casting. The shear-band instability index (SBI) is introduced to explain the effects of the size and aspect ratio on the plasticity of the metallic glass. Stable propagation of shear bands can be achieved for the specimen with small size and low aspect ratio, resulting in a size and aspect ratio dependance of plasticity