What should be the area of ​​a freestanding gas boiler?

Good afternoon, Vladimir.

1 kW = 0.2 m3, with a ceiling height in the boiler room of at least 2.5 m. Regardless of the boiler capacity, the cubic capacity must be at least 15 m3. 25 kW * 0.2 m3 = 5 m3. So the minimum required 15 m3 suits you. Divide by the height of the ceilings 2.5 m (conditionally, since you did not specify the actual height) = 6 m2.

Be sure to consider the convenience of continuous operation. Provide the necessary space for the hot water heater and the accessibility of each boiler for maintenance. Do not forget about the place of storage of the minimum amount of fuel in a safe distance from the boiler. Not always fulfilling the minimum required space is enough for daily comfort.