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In most synthetic self-assembly processes the size of the final structure grows unbound and is only limited by the number of accessible microscopic building blocks. In comparison, biological assemblies can autonomously regulate their size and shape. One mechanism for such self-regulation is based on the chirality of microscopic units. Chirality induces a twisted geometry of building blocks that is incompatible with long-ranged crystalline packing, thereby stopping the assembly’s growth at a given stage. Chiral self-regulating self-assemblies, based on thermodynamic equilibration rather than kinetic trapping, remain an elusive target that has attracted considerable attention. So far studies of chiral self-assembly processes have focused on non-responsive systems, whose equilibrium points are not easily shifted in situ, which limits their versatility and applicability. Here, we demonstrate stimuli-responsive self-regulating self-assembly. This assembly is composed of chiral and magnetically alignable nanorods, where the effective chirality is modulable by balancing chirality-induced twisting with magnet-induced untwisting alignment. Changing the magnetic field intensity, controls the strength of self-regulation, leading to assemblies whose sizes and shapes are rationally controlled. The described size/shape control mechanism is tunable, reversible, robust, and widely applicable, opening up new possibilities for generating biomimetics structures with desirable functions and properties.