Artboard size units not right
Ai 29.4 / Sequoia 15.4
Create an artboard of 59,4 x 60 cm, add some graphics. Save as .pdf.
Place in InDesign, and see how it is actually 5,94 x 6 cm.
I think Illustrator messes with mm and cm.

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Jan, when you try to create an artboard larger than 227 inches (~5766 mm), Illustrator has to enable 'large canvas' mode — and it warns you about this right in the New Document dialog, with a warning symbol next to OK button.
This mode internally scales the document 10 times, by setting custom UserUnit factor, that is supported by PDF above 1.6 (Acrobat 7.0) — not lower. So, when this PDF is placed into InDesign’s document, it should get read properly... unless you use a PDF format lower than 1.6?
Anyway, when I try to test this — it works as expected. Take a look at these two test files.
Please share the exact details at your side: what settings and presets do you use?