Image j software works well for HR-TEM data analysis because FE-SEM provides the agglomerate size but HR-TEM provides precise particle size due to its higher resolution. Better to perform HR-TEM analysis of your respective sample and then use imageJ software to plot the particle size histogram.

You do not HR TEM, any TEM will do. But your FE SEM is not bad also. The main thing you need to get a monolayer of your particles and avoid agglomeration. You may want to try different solvents and concentrations for drop casting of your powder (for both TEM and SEM). May be even to use some surfactant.

In addition, BSE mode is better for such task, because the detector is coaxial with the beam and pictures looks "flat".

Good luck!

Thanks to all for your valuable suggestion.