The virtual source size of a liquid metal ion source is an order of magnitude larger than the size of the region from which the ions are emitted at the source. This source size has a direct effect on the reduced brightness and, hence, on the performance of these sources. The variation of the virtual source size of a gallium liquid metal ion source as a function of the angular current density at the source has been measured. This was done by measuring the source image size from images of a pencil lead sample taken with an FEI focused ion beam system. The measurements indicate that the virtual source size grows from about 50-80 nm when the emission current increases from 1 to 10 mu A. The experimental data on the virtual source size are compared with the theory on stochastic Coulomb interactions in the source region. On the basis of these measurements the authors show that the reduced brightness deteriorates with an increasing angular current density. The maximum reduced brightness measured was 1x10(6) A/(m(2) sr V).