We discuss the final size of a magnetized superbubble and suggest that its size is determined by the balance between magnetic tension and buoyancy. The former is estimated from the radius of the bubble and the magnetic field in the supershell. The later originates from the difference between the shell density and the coronal gas density inside the bubble. When the buoyancy dominates the magnetic tension, the superbubble is transformed into the so-called chimney. The final size of such a magnetized superbubble prior to the chimney structure is several hundred parsecs. To determine its size, Vallée's law to describe the evolution of the magnetized supershell is essential.