to print self adhesive labels
Have you already tried to File -> New -> Labels and searched under drop downs Brand and Type, whether you find your self adhesive labels (and this way getting your correct paper types and sizes automatically from your selection of label type)?
(PS: Changing paper type and size is under `Format -> Page / Page Style -> Tab: Page` , though it is not the way to deal with labels - check also [LibreOffice Help - Page](https://help.libreoffice.org/6.4/en-US/text/shared/01/05040200.html?DbPAR=SHARED#bm_id3150620))