How large can a single-pane window be?

It simply has to do with not letting yourself be fobbed off as a customer with a simple statement from the company, just because they have to manage a more difficult installation...

You are absolutely right, the one-piece window is nicer. If you build with an architect and assign the trades according to your specifications, that should not be a problem.
If you order from a complete provider, you will only get what they have in their catalog or want to install – regardless of what is available, what would be cheaper, or anything else. After all, the complete provider has calculated his warranty risk and taken into account his experience with his suppliers and their products and services. He will hardly want to tamper with that. In this case, you are not being fobbed off, but have a request outside the delivery program.
It is, so to speak, the difference between a tailored suit and ready-to-wear clothing.

Well, the company gladly accepted the extra charge of €2000 for a front door with double-sided overlay panels or the €2100 for 4 venetian blinds... So I think they can also put some effort into a "simple" window... I have once tasked my construction management to inquire about this...

Our largest one-piece fixed window is 3.52x2.53m. It is located on the side away from the street towards the hillside and the glass alone weighs over 400kg. We will take out a very good glass insurance because this beast can only be (re)placed with a crane from the street 30 meters away.

You see, several factors go into the window planning, including the altitude of your house. This somehow has to do with the filling between the panes, our window manufacturer told us.

But all things that can be handled with an additional charge, I think.

We have two bay windows and each one is 1.88m x 2.33m (dimensions finished installed), completely normal plastic windows, have withstood everything so far (storm) and there are no problems with them.

Best regards
Sabine

Our window is 3m high. Stairwell. It has an aluminum frame and safety glass.

Everything's fine. Of course, the crane has to show up, and the thickness has to be right. The surcharge was also reasonable.