Perhaps enabling the aggressive-resize option will help:
set-window-option -g aggressive-resize
A good overview of tmux options is given here.
When attach screen you may use detach mode
tmux attach -d
with will resize screen after disconnect other clients from the sessions (and someone who used small screen)
Or you can interactively detach by pressing Ctrl-B-Shift-D
Starting with tmux 3.1, the default option for
latest. So all you need to do is upgrade.