Thin-wall metal ultramicro- and nanoelectrodes (UMEs/NEs), especially gold-based ones, are key probes for high-resolution electrochemical microscopy, biosensing, and nanoscale interfacial studies. Yet, their broader use remains limited by fragility, low detection sensitivity, and the lack of scalable fabrication methods. Here, we present a template-assisted, non-self-limited polyol-based growth strategy that realizes single-crystalline, thin-walled Au UMEs/NEs, as well as multifunctional probes, with high yield (>80%). The method provides precise control over electrode dimensions, from sub-100 nm to micron-scale radii while the massively parallel polyol growth step overcomes the key bottleneck in scalable nanoelectrode preparation by reliably producing long, continuous single-crystal metal cores. Structural and electrochemical characterization confirm twinned single-crystal Au cores, seamless Au/glass interfaces, and stable performance. Through direct comparison across radii, we show that smaller electrodes consistently exhibit higher surface reactivities, boosting chemical detection sensitivity. In scanning photoelectrochemical microscopy, these NEs achieve an illumination-dependent spatial resolution of ∼250 nm, <1 pA current sensitivity, a detection limit of ∼11.0 µm, and over 7 h of operational stability. In bulk electrolytes, the single-crystalline electrodes achieve ultralow detection limits down to 79 nm, markedly enhancing the signal-to-noise ratio in nanoscale electrochemical measurements. Finally, we demonstrate growth in double-barrel pipettes for multifunctional probes and extend the approach to Pt NEs. This scalable method overcomes longstanding limitations in NE fabrication, enabling advanced electrochemical imaging and its combination with tip-enhanced spectroscopic methods. The single-crystalline architecture also opens new frontiers in catalysis, interfacial electrochemistry, biosensing, and molecular-scale investigations.