Large-scale, defect-free, micro- and nano-circuits with controlled inter-connections represent the nexus between electronic and photonic components. Through a synergic theoretical and experimental investigation, arrays of parallel ultra-long (up to 0.75 mm), monocrystalline, silicon-based nanowires, and complex, connected circuits are obtained exploiting low-resolution etching and annealing of thin silicon films on insulator. Phase-field simulations, predicting both the general trends and widths for self-assembled nanowires, reveal the crucial role of surface energy anisotropy and, thus, crystal faceting in the stabilization of these structures against breaking. Wires splitting, inter-connections and direction are independently managed by engineering the dewetting fronts and exploiting the spontaneous formation of kinks. Finally, using these structures, field-effect transistors with state-of-the-art trans-conductance and electron mobility are obtained. Beyond the first experimental evidence of controlled dewetting of patches featuring a record aspect ratio of ∼1/60000 and self-assembled ∼mm long nanowires, this approach constitutes a novel promising method for the deterministic implementation of atomically-smooth, monocrystalline electronic and photonic circuits.