Planning guest WC in new construction - How big should it be? (DIN?)

I don't think so at all. If I had so little space and lived with three people, I wouldn't plan a table for 6 people and a lot of walking space around it. I would rather use a small table with an extendable tabletop. And separate the kitchen from the room with a counter. It looks like a loveless holiday apartment. With such a few square meters, I also don't see why the kitchen has to be directly by the terrace, which in Hamburg is not used 12 months a year anyway. Such small floor plans have to be thought about completely differently than just scaled down from larger ones.

Sorry, but just because you are a family of three doesn’t mean you don’t sometimes have guests etc. And honestly, I live alone and love my dining table measuring 180x90 cm and couldn’t imagine having such a small cat table at all.

The upper floor is currently a full storey. It may possibly be changed to a knee wall of approximately 2 meters to have more roof area and thus require slightly less distance to the neighbor. And in turn, this would mean more living space on the ground floor or upper floor, but less in the attic. The planning will show what the golden mean is. The roof slope faces south and north. It is the same roof as that of the other half of the house.

It might still be changed to a knee wall from about 2 meters

I would try to avoid that, especially in the children's room area, because with one window it would result in a belly-height window up to glass height of max. about 160 cm. (Because frames and usually roller shutters are added).

Sorry, but just because you are a family of three, that doesn’t mean you don’t occasionally have guests, etc. And honestly, I live alone and love my dining table measuring 180x90 cm and couldn’t imagine having such a small kitty table at all.

We had a small table (100 x 100) for a long time in our first eat-in kitchen, where we comfortably sat with a toddler and a baby. It could be extended with leaf inserts to 180 x 100. We sat at it with ten people during celebrations.

I would try to avoid that, because especially in the children's room area at one window it would mean that you get a belly-height window up to glass height max. about 160 cm. (Because there are still frames and usually roller shutters added).

Good point. The topic is currently still with our construction companies as a "Would that make sense" question. Either leave it or take these flat tubular windows.