Heat pump, how large should the tank be?

Hello,
we are currently building a single-family house with 180m² of living space in Bavaria.
We are now having an air-water heat pump installed and also a water-bearing stove to additionally generate hot water.

Our construction company said an 800l tank is completely sufficient and this is now planned.

However, another stove builder says that this is far too little, and if it is cold for a few days, we will have problems and will not get really hot water.
He also said that a water-bearing stove only causes unnecessary costs instead of providing any benefits?

Does anyone have experience regarding the size of the tank or the water-bearing stove??

Hello,

...
we are currently building a single-family house with 180m² of living space in Bavaria.
Now we are having an air-to-water heat pump installed and additionally a water-bearing stove to also generate hot water.

Whether an air heat pump is the suitable heat generator at this location should be questioned. The milder the climate, the more effective the air heat pump. It would look quite different in the Lower Rhine region.

...
Our construction company said an 800l tank is completely sufficient and this is now planned. However, another stove manufacturer says this is far too little, that we will have problems if it is cold for a few days and will not get properly hot water.

The storage size must be calculated precisely, otherwise there will be problems. Just have the calculation shown to you.

...He also said that a water-bearing stove only causes unnecessary costs instead of bringing any benefits?

He is not entirely wrong. The stove does provide some kWh, but only under very specific conditions. Whether this "yield" is economically reasonable compared to the additional investment would need to be checked ;)
Air heat pump and water-bearing stove operate at completely different temperature levels to work effectively. Significant compromises in economic success result from this alone.

Best regards