Photon gas and highly degenerate quantum gases are not classical ideal gas at all，and each of them has its own mean momentum from which a generalized thermal wave length can be thus defined，and the usual thermal wave length is an irrelevant concept.For a non-degenerate system，this wave length converges to the usual thermal one form a matter particle.For a degenerate system，the dimension of thermodynamic system must be larger than this generalized thermal wavelength，which leads to a criterion on the minimum dimension of the thermodynamic system.For some typical systems，we use the criterion to give their minimum dimensions.