Why the sizes are different?

This is really very confusing … Explain please!

It’s just that I opened the fbx-file, adjusted the units to meters and just decided to measure …

It’s impossible to guess without your 3dm file.

My guess is the named dimension style you’re using for that horizontal dimension, is defined to use a scale factor, perhaps to convert inches to millimeters.

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Found, thanks! I just chose Model units