FeCo alloy nanowires exhibit high saturation magnetization and tunable anisotropy, making them attractive for self-biased microwave devices. However, their magnetic performance is limited by magnetostatic interactions and multi-domain states that degrade coercivity and remanence squareness. This work demonstrates that nanowire diameter directly governs this magnetic complexity. Using experiment and micromagnetic simulation on electrodeposited Fe₆₇Co₃₃ nanowire arrays, we revealed a sharp transition from multi-domain to single-domain configurations below 30 nm. This transition was visualized by scanning nitrogen-vacancy magnetometry and Lorentz transmission electron microscopy. First-order reversal curve analysis quantifies a significant reduction in inter-wire magnetostatic coupling with decreasing diameter, indicating improved magnetic uniformity. Dynamic simulations show that larger diameters introduce multi-domain resonance modes and enhance inter-wire coupling, leading to complex collective behavior. Ferromagnetic resonance spectroscopy provides key material parameters, including gyromagnetic ratio, Gilbert damping constant, and saturation magnetization derived from an effective field model. These results establish that precise diameter control below 30 nm optimizes individual nanowire properties while suppressing array-level coupling, resulting in enhanced coercivity and a stable single-domain state. As a demonstration, a Y-junction circulator based on these nanowire arrays shows promising microwave performance. This work offers a materials-oriented roadmap for engineering high-performance self-biased magnetic devices.