Efficient contrast agents such as gold nanoparticles (GNPs) are highly desirable for goodperformance photoacoustic (PA) imaging. Theoretical analysis found that, owing to the small size effect, GNPs have a unique microscopic mechanism of PA conversion from photons to ultrasound. Here, by quantitatively modeling the optical absorption, the timeresolved temperature field and thermal expansion based on analytical and finite element analysis method, we obtained the quantitative sizedependent PA conversion efficiency of gold nanospheres/nanorods spanning a comprehensive range of particle size (20150 nm). Results show that, both the plasmonmediated absorption and energy conversion from absorbed laser energy to ultrasound are strongly dependent on the GNPs size, which comes from their sharply increased surfacetovolume ratios. The gold nanospheres and nanorods possess peak size values for maximizing the PA conversion efficiency in the size ranging from 20 nm to 150 nm. Our work gives theoretical guidelines for constructing highsensitive PA nanoprobes through rational optimization of the GNPs size.