Minimum size for a house connection room

That won't be the first house that the gu plans. So it will work out. It seems, that's how it sounds, that you actually don't care much about what happens there, because you have "no idea". Let the gu do it, in the end you'll get a roof over your head.

So what now?!

Construction stop?
Everything doesn't matter, it's all good?
Or just a (partial) demolition?

Too cool, really

Yes, depends. If the technology doesn't fit in, replanning is necessary. Otherwise not. But obviously the OP himself doesn't even really know what and how he has to squeeze everything in there.
I wouldn't know what should be so complicated about it.

Then everything would fit in here for us too without a washing machine and dryer.

I strongly advise you – don’t walk so naively through your construction site. Your attitude might work out, but it often backfires. With your 1.75 wall you will already have problems if you do too much with the electrical work. The standard distribution box is 80 cm. The next larger size is already 105 cm. But of course, we don’t know what kind of electrical setup you want or need. Probably the standard size will be sufficient.

And talk to your company about redesigning the door. It will be closed 99.9% of the time because the room just doesn’t offer anything else, but if something does need to go there, at least you’ll still have storage space for a small shelf on the left.

Yes, you can have problems with the acceptance.

DIN 18 012
5.1 House connection niche, house connection wall, and house connection room shall be planned on the basis of this standard and, if necessary, in coordination with the supply and disposal companies in such a way that all connection devices and, if applicable, the operating equipment provided there can be installed and maintained properly.
5.2 When determining the location within the building, sound insulation according to the standards of the DIN 4109 series must be observed.
5.3 The arrangement of the house connection niche and the house connection wall or the size of the house connection room shall be planned so that a service and working area with a depth of at least 1.20 m is available in front of the zone assumed to be 30 cm deep for the connection and operating equipment.
...
6.2.2 Appropriate structural measures must be taken for the continuation of the lines from the house connection niche (e.g. slots, empty conduits, cable ducts), paying particular attention to the statically effective elements (e.g. lintels, beams).

I didn’t approve anything because I have no idea about such things.

You signed the building application. You are the builder.

But how did the general contractor get the building permit if everyone says the room is way too small?

Not the general contractor, you got the permit.
And why? Because something like that may not be checked, since there are individual solutions for the house suppliers. After all, there are heating systems including storage tanks that are no bigger than a wall cabinet.

You could also have asked here whether the corridor with approx. 1 meter width, a total of 4.11 sqm, is sufficient. Users here would be divided, because of course it works somehow, but only barely.
Is this a weekend house or a residential house for primary residence?
Ultimately, as the builder, you also have to think for yourself and question some things. After all, you don’t build just to put the sofa back in front of the TV. You also have to somehow store your everyday items in your new rooms and be able to do your daily work.
For me, it was definitely clear that I could not store all the household consumables in a technical hole of less than 6/7 sqm. Accordingly, it was built.

You signed the building application. You are the client. Not the general contractor, you received the approval.

I did not submit / sign a building application. The general contractor did that.

If that is really the case, you don't have a GU but a BT, then none of this really concerns you and in the end you are to be handed over a house free of defects.