We report seed-mediated synthesis of large (up to 180 nm) Au nanoparticles (NPs) for size-optimized surface-enhanced Raman scattering (SERS) at Pd overlayers. Our results reveal that overmuch produced electrolyte during synthesis may destabilize the growing particles, making them failure to grow freely. This understanding guides the synthesis of large Au NPs with a large range of sizes, thereby allowing exploiting their size-optimized properties, including borrowed SERS achieved at ultrathin, pinhole-free Pd overlayers. The high quality borrowed SERS achieved is important for in situ extracting of vibrational information for species on catalytic Pd surfaces. (C) 2015 Elsevier B.V. All rights reserved.