Using XUbuntu 18.10 (but suspect same behaviour on earlier versions, now that I've noticed it!):
Open a terminal - default geometry is 80x24 as expected.
In that terminal, open a new tab (ctl-shift-T). Now geometry on both original terminals is maybe 96x24, sometimes 97x24! (As reported by 'tput cols' and/or echo $COLUMNS)
How can I get it to remain at 80xYY rather than 96xYY?
(Yes, have looked at the documentation, in ~/.config/xfce4/terminal/terminalrc and in /etc/xdg/xdg-xubuntu/xfce4/terminal/terminalrc, in the Preferences settings, etc!)