  Atom probe tomography (APT) is routinely used for analyzing
property-enhancing particles in the nanometer-size range and below, and plays a
prominent role in the analysis of solute clusters. However, the question of how
well these small particles are measured has never been addressed because of a
lack of a reliable benchmark. Here, to address this critical gap, we use an
approach that allows direct comparison of APT and small-angle (X-Ray)
scattering (SA(X)S) performed on the same material. We introduce the notion of
an effective spatial resolution for the analysis of particles, which,
importantly in this context, is very different than the technique's inherent
spatial resolution. This effective resolution is highly specific to the system
being considered, as well as the analysis conditions. There is no hard limit
below which the technique will fail, but particles with a radius of order of
\textapprox $2\sigma=1nm$, i.e. \textapprox\num{250} atoms cannot be accurately
measured, even though the particles are detected. This thorough metrological
assessment of APT in the analysis of particles allows us to discuss the pulse
spread function of the technique and the physics underpinning its limits. We
conclude that great care should be taken when analysing solute clusters by APT,
in particular when reporting particle size and composition.
