Designing more reliable irradiation-resistant material is an essential aspect of nuclear engineering. Dislocation channel formation is one of the most important catastrophic failure origins. Since dislocation channels are typically 50~200nm wide, an intriguing question is that if the sample size decreases to the submicron scale, and becomes of the same order of the characteristic length, then how does irradiation influence plastic flow localization? We report the first systematic three dimensional Discrete Dislocation Dynamics (3D-DDD) observations about the size effect on dislocation channel formation. Because of the high density of radiation-induced defects, massive simulations of discrete interactions between dislocations and all radiation defects is computationally prohibitive. To overcome this computational difficulty, we developed a hybrid continuum-discrete model for the collective dynamics of dislocations in dense irradiation defect field. To quantify plastic flow localization, we condense the complex 3D deformation information into some easy-to-handle parameters to describe the flow localization. 3D-DDD simulations reveal that with the reduction of external size, the flow localization mechanism transitions from irradiation-controlled to an intrinsic dislocation source-controlled, and the spatial correlation of plastic deformation decreases due to weaker dislocation interactions and less frequent cross slip, which manifests itself through thinner dislocation channel. A discrete dislocation source activation model coupled with a cross slip tuned channel widening model is developed to reproduce and explain this transition. This finding has implications to the design of radiation-resistant materials.