Size effects and their consequences in the thermal stability of the magnetization of the micro-sized MnAs/GaAs(100) ribbons were studied by magnetic force microscopy. We found out that the orientation in which size is reduced plays a key role in the magnetic configuration and thermal stability of the micro-sized patterns. On the one hand, when reducing the size in the [0001] α-MnAs direction, the system shows an improvement in the thermal stability of the remanent magnetization. On the other hand, when the size is reduced in the [11-20] α-MnAs direction, the system goes through a magnetic reconfiguration from in-plane magnetized to out-of-plane magnetized, which also improves the thermal stability.