How do I change margin sizes to a minimum?

To maximise the paper for printing

Format → Page → Page tab. That will change the formatting of the page style applied to the cursor position.

(In addition to what @RGB-es already posted:)
If you set all the page margins to zero in the dialog opened on >Format>Page... and then click OK, LibO will check if the printer chosen under >File>Printer Settings... can print to the paper edges or what minimum margins it needs. If there are minimum margins your OK will bring up a prompt asking you if you want to ignore the conflict. If you now answer NO, the actual page margins will be set to the printer’s minimums.

This answer is taken from my experience. I do not know a respective specification. No guarantee of any kind!)