Minifridge sized which could definitely fit inside almost any car especially if the power is converted using hydrogen fuel cells (electric motors). https://mcphy.com/en/equipment-services/electrolyzers/ To put into perspective, the Tesla Model 3's battery pack puts out 4.5 kw at peak. The whole assembly (which would require water, storage tanks, bla bla bla) is just too big for a car. Optimally, the hydrolysis (electrolyzer) is at your home where you refuel using literal tap water and ultra-filters. Your costs would be replacing the filters. The concept existed to fuel homes completely with no power grid connection including powering your car (could be electric or hydrogen fuel cell). Concept and it was put into practice. Australian company had working prototypes about 10 years ago. Wonder what happened to them?