Construction costs are currently skyrocketing

Exactly that is the case, but I will try to have the conversation again tomorrow and ask for an "explanation" of the sizing.

Very good idea and please definitely also ask why he wants to heat with the heating rod. That is complete nonsense. And regarding the sizing and for your orientation... I have an ILM 2-7 for 140 sqm plus basement with underfloor heating and insulation standard not quite at KFW55. I believe my heat pump never runs above 4 kW at outdoor temperatures above 0 degrees.

The topic of heating is also very slow for us. During the plumbing inspection, the plumber mentioned that we would be getting a Stiebel Eltron LWZ 5 S. Today his company installed the heating and it is a larger model: LWZ 8 CS Premium. We have 200sqm of living space and 50sqm of usable/basement space (half-basement, garage in the building).

The topic of heating is also very slow for us. During the plumbing inspection, the plumber mentioned that we would get a Stiebel Eltron LWZ 5 S. Today his company installed the heating and it is a larger model: LWZ 8 CS Premium.
We have 200 sqm of living space and 50 sqm of utility/cellar space (partly basement, garage in the building).

He installed a different one than ordered without asking?
Single contract?
And is it supposed to cost more then?

In 2020, the base price for the Flair 152 RE was somewhere around 200-220k (about 200k was, I think, a promotion back then and about 220k otherwise). Online we are shown 278k.... a "nice" development.

You have to specify the region for it. Between the MD or HAL region and M or S, there is often a 20-25% difference at Town & Country. I saw the 152 RE for 183,000 EUR in mid-2019, and at the end of 2019, the base price was 189,000.

He installed a different one than ordered without asking?
Single contract award?
And is it supposed to cost more?

We are working with a general contractor. Everything is going great. Our contract specifies the Stiebel Eltron LWZ 5 S or the LWZ 8 CS Premium. During the plumbing inspection in the shell construction phase, I asked the boss. He told me that the heating system would "definitely not be oversized" and it would "probably be the 5 S." Since they also do the heating for a large prefab house provider, they have enough units of both in stock.
No additional costs will arise.

In what kind of Bollerbü do some people live? If the electricity price stays at 50 cents or more in the long term, you will soon be unemployed

This is even more true for the gas price... I am curious how the ovens in cement, brick, or glass production are supposed to be heated if not with gas.
Also process heat... VW, for example, burns gas to generate electricity and uses the waste heat as process heat. If that is no longer possible, VW will have to generate its process heat with electricity. Where is that supposed to come from in such quantities?

PS:
And how will the baker heat his oven in the future? With a heat pump?
Come on, forget it :D