Did some experiments, and seems to me that LibreOffice will always put your graphics on your page format (and I'm not sure that this is a wrong choice, anyways).
But I found that if you use File → Export as PDF you can check Selection
in Range
section to make it export only selected elements, however it will always put them on your original page format.
But after you created your PDF file you can automatically crop away blank areas using cli tools like pdfcrop
, as described here: https://askubuntu.com/questions/124692/command-line-tool-to-crop-pdf-files
For example, you can crop illustration.pdf
leaving a 5 pt margin with:
pdfcrop --margin 5 illustration.pdf
There are also GUI applications that can crop PDF files, such Briss. For similar tools see also https://stackoverflow.com/a/17406976/793641
On Mac OSX you can even use Preview.app
to crop PDF files. Just use the rectangular selection tool from menus, select the area you need and then use the crop tool from the same menu.