We demonstrate a new route to the precision fabrication of epitaxial semiconductor nanostructures in the sub-10-nm size regime: quantum-size-controlled photoelectrochemical (QSC-PEC) etching. We show that quantum dots (QDs) can be QSC-PEC-etched from epitaxial InGaN thin films using narrowband laser photo-excitation, and that the QD sizes (and hence bandgaps and photoluminescence wavelengths) are determined by the photo-excitation wavelength. Low-temperature photoluminescence from ensembles of such QDs have peak wavelengths which can be tunably blue shifted by 35 nm (from 440 nm to 405 nm), and have linewidths which narrow by 3 times (from 19 nm to 6 nm).