Create new tmux session from inside a tmux session

I figured it out (and had it pointed out to me).

tmux attach || tmux new

tmux limits the dimensions of a window to the smallest of each dimension across all the sessions to which the window is attached. If it did not do this there would be no sensible way to display the whole window area for all the attached clients.

The easiest thing to do is to detach any other clients from the sessions when you attach:

tmux attach -d

Alternately, you can move any other clients to a different session before attaching to the session:

takeover() {
    # create a temporary session that displays the "how to go back" message
    tmp='takeover temp session'
    if ! tmux has-session -t "$tmp"; then
        tmux new-session -d -s "$tmp"
        tmux set-option -t "$tmp" set-remain-on-exit on
        tmux new-window -kt "$tmp":0 \
            'echo "Use Prefix + L (i.e. ^B L) to return to session."'
    fi

    # switch any clients attached to the target session to the temp session
    session="$1"
    for client in $(tmux list-clients -t "$session" | cut -f 1 -d :); do
        tmux switch-client -c "$client" -t "$tmp"
    done

    # attach to the target session
    tmux attach -t "$session"
}
takeover 'original session' # or the session number if you do not name sessions

The screen will shrink again if a smaller client switches to the session.

There is also a variation where you only "take over" the window (link the window into a new session, set aggressive-resize, and switch any other sessions that have that window active to some other window), but it is harder to script in the general case (and different to “exit” since you would want to unlink the window or kill the session instead of just detaching from the session).