How can I record the DPM particles for individual size groups?
You can record the particles for individual size groups by the following way.
Display-> particle Tracks
Report Type : Step by step
Report to : File
Color by : Particle variables (Particle Diameter)
This will write the particle tracking history in a file. The last column gives the particle diameter. From this, you can find out the number of particles in a size group. In this way you can find out the particles in a density/mass/temperature etc group by selecting these variables in the 'Color by'.