The easiest is Export… , choose PNG (and tick Selection I presume). When you click Save you are taken to a new dialog where you can set resolution. Before you set the resolution take a note of the size because increasing resolution decreases size so you after you set to 300 dpi got into one of the sizes and type in the original value, the other will change to match, OK out. You can now convert your your png to a TIFF in a graphics editor. I don’t know why the TIFF export doesn’t have the sizing dialog, everything else seems to. I always use png so haven’t noticed before. Cheers, Al
Another option is considered here but uses Inscape too: https://superuser.com/questions/686793/exporting-high-quality-png-from-libreoffice-draw