<p>Methylammonium lead tribromide (MAPbBr₃) single crystals (SCs) have garnered considerable attention for room-temperature γ-ray and X-ray detection. However, as one of the most easily grown perovskite SCs, scaling up their size often compromises crystal quality. Here, we present a strategic precursor stoichiometry engineering to grow inch-sized, high-quality MAPbBr₃ single crystals via a constant-temperature evaporation method. We demonstrate that constructing a robust electrical double layer through organic cation modulation effectively stabilizes the colloidal precursor. This is achieved by synergistically suppressing MA⁺ deprotonation while promoting MA⁺ adsorption as counterions on the [PbBr_<i>n</i>]^2−<i>n</i> complexes, which collectively strengthens interparticle repulsion and raises the nucleation barrier. This multifaceted approach yields in MAPbBr₃ SCs with lateral dimensions up to 2 inches and an exceptional X-ray diffraction rocking curve full width at half maximum (FWHM) of 0.0093° at the (002) face, consequently enabling spectroscopic-grade γ-ray detection, achieving energy resolutions (ER) of 8.4% for the ⁵⁷Co source (122 keV) and 11.1% for the ¹³⁷Cs source (662 keV), along with a high X-ray sensitivity of 1.65 × 10⁴ µC Gy⁻¹ cm⁻². This work paves the way for the practical application of MAPbBr₃ SCs in high-performance γ-ray and X-ray detection.</p>