How do i make or customize the title bars size small in Ubuntu themes?
In general, using apps which do not use CSD can help you reduce the titlebar size. Usually the MATE apps use traditional menus instead of CSD.
For example, you can use mate-terminal instead of gnome-terminal. GNOME Terminal uses CSD, which takes up a lot of vertical space, whereas mate-terminal uses a traditional menu.
sudo apt install --no-install-recommends mate-terminal
MATE Terminal
Then change the default terminal with the following command, and choose mate-terminal there.
sudo update-alternatives --config x-terminal-emulator