This refers to LO 5.x in Linux (Mint 18.3, based on Ubuntu Xenial). Things may be different in other contexts.
Specifically it is the apps Writer and Calc which concern me. The scrollbars seem to be about 5 px thick. This is far too thin for my giant monitor (43", 4K resolution).
I have found a way to edit the .css file used in Ubuntu /usr/share/themes/Mint-X [or YOUR THEME]/gtk-3.0/gtk-widgets.css to change the scrollbar thickness of other apps… but at the moment LO apps are not cooperating.
This should be a configurable option under Tools → Options → LibreOffice → Accessibility: sight-impaired people and people with dexterity problems, as well as people with giant screens, are annoyed by preposterously thin scrollbars. This is a pretty simple requirement to understand.