Time-resolved laser-induced incandescence (TR-LII) measurements have been performed inside the combustion chamber of a heavy-duty diesel engine running at low load and with regular diesel fuel. The LII traces were interpreted in terms of primary particle sizes, comparing two different assumed particle-size distributions: a mono-disperse and a log-normal distribution. The initial temperature of the particles (immediately after the laser pulse) is estimated by two-color pyrometry. We conclude that the initial temperature of the particles is not very critical for the outcome of the fitting procedure for the (mean) radius. Under the high-pressure conditions in the engine, the time dependence of the LII intensity contains sufficient structure to allow retrieval of details of the particle-size distribution.