The present study investigates the primary atomization process of a liquid oxygen/methane powered liquid rocket engine. The study is based on a multi-scale description of the primary liquid ligaments and leads to the determination of their size distribution according to the distance to the injector. The image-based method is improved by using a sub-pixel analysis and a refined surface area measurement technique. Applied to experimental backlighting images obtained at ONERA’s MASCOTTE rocket combustor, measurements report that the ligament size distribution increases in average and in dispersion as the distance from the injector increases. The distance at which these liquid structures contain the greatest amount of liquid (and produce therefore the greatest number of drops) is also identified. These results are valuable and may be helpful for the development of numerical simulation models to correctly account for the primary breakup process.