Time-resolved transmittance spectroscopy was performed in the wavelength range of 610 or 700 to 1050 nm on phantom parallelepipeds and bone tissue cubes of different sizes. The data were best fitted with solutions of the diffusion equation for a laterally infinite slab and for a parallelepiped to investigate how size and optical properties of the samples affect the results obtained with the two models. Monte Carlo simulations were also performed to support and help with the interpretation of the experimental data. The parallelepiped model performs much better than the infinite slab model for the estimate of the reduced scattering coefficient and, even more, the absorption coefficient. It can profitably be used to quantify the optical properties of biological tissue samples and to derive information such as tissue composition, when small volumes are involved.