We study the trend of structural stability and magnetic moment for MnX (X=N, P, As, and Sb) binary compounds in the NiAs and zinc-blende structures through first-principles spin-density-functional calculations. The exchange splitting and magnetic moment are generally lower in the stable structure, which corresponds to the NiAs structure for MnP, MnAs, and MnSb whereas the zinc-blende structure for MnN. With the exception of MnN, we find the increasing trend of the stability, exchange splitting, and magnetic moment of the ferromanetic state with the anion size along the series MnP, MnAs, and MnSb. Since the N p level is much lower than other anion p levels, the Mn-N bond is more ionic, and thus MnN favors the zinc-blende structure with a lower coordination number. For MnN and MnP with small and light anions, the ground state is an antiferromagnetic state, while a ferromagnetic state is more favorable for MnAs and MnSb, which have larger equilibrium volumes and thereby reduced p-d and d-d couplings for the majority spin channel. Due to the volume and different bonding effects, MnAs and MnSb show a large exchange splitting for the d states and exhibit a nearly half-metallic behavior. The large volume increases the anion p-type character near the Fermi level, and thus the ferromagnetic state is stabilized through double exchange interactions.