A good size measurement method should meet at least three criteria. These are rotational invariance, reproducibility and embody overall shape description (elongation / flakiness or angularity). According to these three criteria, this paper analyzes and evaluates several existing methods of solid particle measurement, such as Chord sizing, (multiple) Ferret diameter, equivalent circle, maximum diameter and equivalent ellipse etc. in image analysis. Based on the analyses and evaluations of the existing methods, we propose a new method – best-fit rectangle for size measurement that satisfactorily meets the criteria of rotational invariance, reproducibility and shape description.