The microstructural analysis of melt-cast processed Bi₂Sr₂CaCu₂O_x reveals the existence of grain boundaries on two different length scales. Plate-shaped crystal grains of approximately 10 mu m length are textured along the c direction on a scale of 200-300 mu m, forming bundles of arbitrary orientation. To investigate the impact of the grain boundaries within and between bundles on the critical current density the authors have analysed the variation of the magnetic hysteresis with the sample size using different powder fractions and bulk material. It could be shown that the boundaries between the platelets within a bundle limit the overall transport current density in bulk material, whereas the boundaries between bundles do not further reduce its value. The results are in agreement with a TEM analysis of the grain boundaries.