Atlas Vernier Start Tank

In addition to its two outer booster engines and its center sustainer engine, the Atlas missile (and subsequent space launch vehicle) had two smaller vernier engines (LR-101 engines) to provide roll control and fine attitude control.

During mainstage, the verniers used propellant from the main tanks (provided under pressure from the sustainer engine's turbopump). However, the verniers were started prior to sustainer mainstage. During bootstrap, the vernier engines' propellants were supplied by two smaller auxiliary tanks, as displayed here.

Or, perhaps, "as similar to displayed there:" The ID plate on the tank actually indicates that it's a helium container, rather than a propellant tank.

Nonetheless, the Atlas verniers were equipped with similar tanks. The LOX tank held 4,140 in³ (which is about 17.9 gallons and would be a sphere about 19.9" in diameter) and the fuel tank held 2,920 in³ (which is about 12.6 gallons and would be a sphere about 17.7" in diameter). Both tanks and a nominal operating pressure of 600 psi.