Bulk-mechanical and micro-mechanical testing have been performed to investigate the deformation mechanisms in austenitic Fe-22wt%Mn-0.6wt%C TWIP steel single crystals with a stacking fault energy (SFE) of 23 mJ/m2. Compressions of micro-pillars oriented for deformation-induced twinning and for perfect dislocation glide have been conducted for micro-pillars with diameters in the range of 0.6 μm to 4 μm. The same size-dependence of the critical resolved shear stress was observed for both orientations. Further, we found that the critical resolved shear stresses of micro-pillars oriented for deformation twinning and micro-pillars oriented for dislocation glide were similar. The elasto-plastic transition in micro-pillars oriented for deformation twinning did not involve twinning, and dislocation-dislocation interactions were a necessary precondition for twin formation. Both, calculations and experimental observations imply that the elasto-plastic transition of the micro-pillars is due to dislocation glide, independent of the orientation. The calculated critical micro-pillar diameter for size-independent plasticity was about 7.6 μm. Partial dislocation-mediated formation of twins and e-martensite was observed in micro-pillars oriented for twinning by transmission electron microscopy (TEM).