We use a double nanohole (DNH) optical trap to quantify the size and concentration of nanoparticles in solution. The time to trap shows a linear dependence with nanosphere size and a -2/3 power dependence with nanosphere concentration, which is in agreement with simple microfluidic considerations. The DNH approach has size-specificity on the order of a few nanometers, which was used to selectively quantify particles of a single size within a heterogeneous solution. By looking at individual trapping events, it is in principle possible to extend this approach to the ultimate limit of a single particle concentration, while also being able to operate at high concentrations in the same configuration. In addition, the DNH trap allows us to hold onto individual particles and thereby study constituents of a heterogeneous mixture. By repeating the trapping measurements on spherical particles of different refractive index, we found that the transmission step that indicates trapping scales empirically with the Clausius-Mossotti factor. This approach may be applied to several sensing applications, such as in the study of virus populations, where concentrations vary over many orders of magnitude.