Hibiscus sabdariffa calyxes are rich in anthocyanins and polyphenols, giving them interesting antioxidant properties. Powdering and fractioning by sieving improve the extractability of these biomolecules and allow producing powders presenting targeted functional properties. In a view to elucidate powder reconstitution steps, morphogranulometry, conductimetry, and microscopy methods were employed to characterize particle size and shape evolutions during reconstitution and their link with biomolecule release. During reconstitution, water diffusion into particles led to rapid polar molecules solubilization: roughly 20 s for fine particles (11–85 μm); 1.5 min for larger particles. Continuous swelling and particle dissociation simultaneously occurred, followed by fragmentation. Particle fineness and porous structure favored the penetration of water, then facilitated the swelling and the dispersion steps, which enhanced the anthocyanin release. These bioactive molecule-rich fine powders revealed higher total antioxidant power (PAOT) (289.40 ± 1.93 PAOT score/g) than coarse powders (207.59 ± 4.96 PAOT score/g) containing more fibers.