I have inserted a document via a screenshot, but it covers ca. 1.3 pages, I want to compress it to match exactly one page! ???
And setting Width / Height at Right Click -> Properties -> Tab: Type -> Category: Size doesn’t work for you? (Seems to me you want to change size and not to compress the image)
Please edit your question to be more descriptive (don’t use an answer, this is additional information). Do you Insert>File (for an .odt document) or an image (.jpeg, .png or other format)?
Don’t forget to mention your OS and LO version.