Abstract We evaluate different approaches to determine the diameter of silicon nanoparticles in the regime of ultra-small particle sizes ( d < 5 nm ). The nanocrystals are fabricated using a plasma-enhanced chemical vapor deposition (PECVD) process and are embedded in a matrix of SiO₂. For characterization, different experimental techniques can be used, ranging from optical measurements such as photoluminescence or nonlinear optical response over transmission electron microscopy. An extensive analysis leads to a high-precision size determination and a good agreement between most of the techniques. Apart from the particle size, additional information can be derived depending on the method of choice, such as exciton fine structure splitting energy, width of the particle size distribution etc. While the nonlinear signal shows the expected enhancement for smaller nanoparticle sizes, it turns out to be inadequate for determination of particle sizes with a high accuracy.