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

You are absolutely right, there can definitely be a much more detailed discussion about this and many things to consider. The semi-detached house will have a footprint of 8.50 x 7.70m. 2 full floors, 1 attic. In my search for inspiration in the floor plan forum, I was not successful. It seems like hardly anyone plans under 140m². I am reconsidering whether to let the first professionally drawn drafts be discussed in the forum.

The info about the non-existent DIN standard for room width is good, thank you very much.

A few notes:
- On the upper floor, a kind of broom closet with the same dimensions as the guest WC is planned at that location.
- We and the close relatives are slim enough for a narrow bathroom (our current bathroom in the rental apartment is even narrower than 1 meter at the WC location. I have never noticed that negatively over the years.
- The house will have 3 residents. We plan to feed our son in such a balanced way that he will never become too broad for the guest WC.
- The area gained by a narrower guest WC could be invested 100% into the living/dining area (and every centimeter really counts there)
- There are hardly any other saving options
- A door on the long wall of the guest WC would be great, but is not possible
- No one likes mini washbasins where you have to wash your fingers one by one, but some compromise between washbasin size and room size will certainly be found

In the upper floor, a kind of broom closet with the same dimensions as the guest WC is planned at that spot.

Yes, then a bathroom at the top and two private rooms on the left side?! There isn't much space for development, that's true. Do you have a basement? P.S. You have to clean the toilet too, so you should also be able to reach the wall behind it.

Our plumber said that one should be able to lean forward while sitting without hitting the sink. Otherwise, there is a risk of getting a bump or a laceration. And the door should open outward so that in an emergency the person inside the room does not block the door.

I am reconsidering whether to discuss the first professionally drawn drafts in the forum.

I would recommend that to you. Even if you eventually decide on the original draft, you will have consciously made that decision and know the disadvantages. And not just because the architect suggested it and you, as a layperson, do not see the disadvantages. Regarding your sketch... with the shifted toilet, you need the bathroom to be wider so that your right hand does not constantly hit the wall. I would move the sink closer to the door and place the toilet in the middle. A small sink is sufficient for a bathroom, no problem. And with small houses, it is doubly advisable to consider a basement; for example, this frees up the technical room on the ground floor. You can also reduce storage space in the kitchen, etc.

I assume that the combination of a small plot with a semi-detached house and a small building window was not chosen to create a challenge, but for financial reasons. Coming with a basement... well.

I speculate,

I don't, because it says here, see:

we are currently planning a semi-detached house. The building plot is very small

You don't consciously choose the building plot. Better to ask than to speculate ;)