High-temperature sodium-nickel chloride (Na-NiCl₂) batteries offer a competitive solution for stationary energy storage due to their long-term stability, high energy efficiency, and sustainable raw materials. However, scaling up this technology faces challenges related to the costly integration of tubular Na-β''-alumina ceramic electrolytes into hermetically sealed battery cells. Alternative cell designs with a planar Na-β''-alumina ceramic electrolyte have been a focus of research for many years, and a series of achievements were made on cell design, on reduction of the operating temperature, and on the analysis of electrochemical reaction mechanisms. However, the data presented in these reports was derived from laboratory-scale cells with small area (1-5 cm²). To date, there has been no research conducted on enlarging planar cells to an economically viable size. Here we report the fabrication of large planar Na-β''-alumina electrolytes and their integration into planar Na-NiCl₂ cells with 90 cm₂ active area and >7 Ah capacity. Our cell design enabled cycling at 300 °C for three months, transferring a cumulative capacity of 323 Ah. We discuss design and engineering considerations for large planar high-temperature cells emphasizing the need for cell stacking to compete with tubular Na-NiCl₂ batteries in terms of mass-specific energy.