The formation of gas bubbles surrounding laser heated copper nanoparticles in superfluid helium at 1.7 K is observed. Because of the effective light capture by these plasmonic particles and the subsequent heat transfer into the liquid, such bubbles grow within 3 mu s to tens of micrometers in size. The Schlieren imaging technique is used to determine the spatial distribution of the nanoparticles in the liquid, and the gas bubble radii are related to the parent nanoparticle size. The presented liquid-phase particle size analysis is validated against atomic force microscopy measurements of nanoparticles deposited from the liquid onto a solid substrate.