Prior studies on shape memory alloys have identified size effects on the superelastic, i.e., stress-induced, hysteresis of martensitic transformations. However, literature on thermally induced transformations and size effects upon stored elastic energy is rather limited. In this work, a complementary sample size effect on the stored elastic energy of the transformation, and its effect on variant selection, is elaborated. Shape memory alloy microwires of a CuAlMnNi alloy are drawn with diameters varying between 45 and 255 mu m and processed to obtain bamboo grain structures, where the grain boundaries lay almost perpendicular to the wire axis. Calorimetric and thermomechanical analyses of the microwires establish a decreasing contribution of stored elastic energy to the free energy of martensitic transformation as the wire diameter is reduced. This in turn affects the transformation ranges and macroscopic strain generated in constrained thermal cycling. The effect is shown to be associated with a decrease in number of interacting martensite variants as well as relaxation on free surfaces. The presented results indicate that thermal actuation of lightly biased SMA wires is enhanced in finer wires.