Mesoscale Magnetostructural Phase Separation in Fe-deficient Fe₅GeTe₂

2D Van der Waals ferromagnet Fe_5-xGeTe₂ (F5GT) is promising for spintronic applications due to its high Curie temperature, layered structure, and ability to host complex magnetic textures. However, the origin of its sample-dependent magnetic anisotropy remains unclear, hindering control of its magnetic behavior. Here, spatially resolved cryogenic scanning transmission electron microscopy (STEM) is used to correlatively map magnetism, lattice structure, and chemistry across atomic-to-micron scales. This is revealed that only mesoscale, not nanoscale, inclusions of a Fe-deficient secondary phase significantly modify magnetic behavior, establishing a previously unrecognized critical length scale. This phase separation, induced by quenching, leads to in-plane magnetic anisotropy, while slow cooling confines separation to a few nanometers and preserves out-of-plane anisotropy. These findings reconcile prior inconsistencies and establish a predictive framework for tuning magnetism in F5GT through thermal processing, with broader implications for controlling anisotropy in other 2D magnetic materials.