How large can a single-pane window be?

Of course, such windows can basically also be made as a single piece. This just has to be planned and paid for construction-wise. I think the discussion depends on these two factors.

Well, when I configure the windows online at the relevant window suppliers, the single-part window is even cheaper than the two-part one...

The window itself maybe, but also the transport and the professional installation with a warranty? Total costs are interesting. Individual prices only matter if you also buy individual parts.

Depending on the exposed location, the wind load argument is not nonsense and, assuming the one-piece design, "valid" - presumably there is a communication problem, and the two-piece design was not recognized by the window manufacturer - possibly because the general contractor wanted to save money and requested it without a mullion?

I think you are talking a bit off topic.

The general contractor requested a two-part window with a mullion. Or how else should a division be realized?

However, we would like it to be single-part.

You are kind of missing the point, I think.

No, you said it was the one from the avatar picture - and there it has a center post.

But we would like it to be one piece.

Then the window fitter is right, I say as a window manufacturer, where complaints were a matter for the boss. The crane is an additional factor, no one hauls the scaffolding up in one piece. So: installing without a center post is more expensive and if you were my customer, with an extra two weeks’ waiting time due to the cancellation policy regarding the warranty exemption. To gaze from an in-house balcony onto the street without a post? – you don’t otherwise allow yourself anything.