This work reveals the rules of using figures of merit on selecting efficient plasmonic nanoheaters. Here, a size dependence of plasmonic nanoparticle optical heating was disclosed. The continuous laser heating of gold nanospheres is evaluated exploring a theoretical approach and thermal lens spectroscopy, which allowed identifying micro-degree temperature changes of laser heated colloidal gold nanospheres of varying sizes. Our findings indicate that the temperature of photoheating colloidal particles rises according to the Joule number values. The optimal gold nanospheres diameter for efficient colloidal laser heating was identified to be 50 nm. Moreover, we demonstrated that long-time domain measurements of colloidal sample enable the identification of single-particle intermediate steady-state temperature. Considering a single particle, nanospheres with diameter larger than 80 nm exhibit superior heating performance than smaller particles, revealing that the optimal particle size for single-particle optical heating applications is different from the optimal particle size for collective heating of nanoparticles. Our results pave the way for the rational use of plasmonic nanoheaters in photothermal applications.