We study a binary mixture of short and long parallel hard rods in the smectic A phase to understand the role of length ratio (weight ratio) in the formation of three different types of ordering which were observed in binary mixtures of helical polysilanes. Using Onsager's second virial theory of parallel hard rods, and modeling the short and long polymers as hard cylinders of different lengths (L-1 not equal L-2) and same diameters (D), we show that the normal smectic A phase (S-1) forms for length ratios (l = L-2/L-1) between 0.58 and 1, the microsegregated smectic structure (S-2) takes place for 0.32 < l < 0.56 and 0 < l < 0.39 in short-rod-rich and long-rod-rich phases, respectively, and the two-in-one ordering (S-3) is stable only in mixtures rich in long rods for 0.33 < l < 0.58. These results are in very good agreement with the available experimental data of binary mixtures of polysilanes [Okoshi et al. Macromolecules 2009, 42, 3443]. In addition, the theory predicts the existence of a partially microsegregated smectic structure (S-4), in long-rod-rich mixtures, for 0.3 < l < 0.39. Our theoretical results show that two or even three smectic order parameters should be measured, for all types of smectic phases, in order to obtain the correct positional distribution functions from the X-ray diffraction pattern.