The mechanical properties of natural fiber reinforced composites depend on many factors, fiber dimensions being one of the most important. During compounding, fibers’ bundles and elementary fibers are breaking, and quantification of the fragments length, width and aspect ratio is therefore crucial to predict and understand the mechanical properties of a composite. The lack of an international standard for the measurement of the biomass morphology opened the way to the development and the utilization of several different methods. In this work we investigated the advantages and the limitations of three techniques that are commonly employed to measure lignocellulosic fragments’ dimensions, i.e. high resolution 2D scanner, automated dynamic fiber analyzer, and X-ray microtomography. Two types of fibers of different morphology and composition, hemp and miscanthus, are chosen. Composites are prepared by using a laboratory-scale co-rotating twin-screw extruder, at two feed rates and constant screw speed. We show that the analysis of the morphology of lignocellulosic fragments is still complex and does not allow an objective estimation of the sizes. Each method has its own advantages and disadvantages. Overall, the three methods are complementary and the use of each depends on the goal of the work.