In LibreOffice (Calc) 6.4 the scrollbar is too small/thin. How do I increase the size? The current size is very hard to use on a display size 1920x1080. I am running Linux Mint 19.3 xfce.
The answer depends on the display manager and how ti handles widgets. So, please, edit your question (this is additional data, not a solution; therefore don’t use an answer) to tell your LO version, OS and eventually the desktop manager.
About LO, check what is written after VCL:. It should read gtk3. If it does not, install package libreoffice-gtk3 (or similar name in your distribution). If it is already installed, you can enlarge the scrollbars (in all applications) by modifying the GTK+ CSS style sheet. This is explained on the GTK+ site, but the locations of the master file and of the user-specific override file vary with distribution.