I often attach to a tmux sessions from multiple computers. tmux will limit the terminal size to the smallest client connected. The smaller terminal client must be detached to not be limited in the larger terminal.

List the clients.

$ tmux list-clients   
/dev/pts/2: 0 [90x49 screen] (utf8)          
/dev/pts/6: 0 [274x69 screen] (utf8)

Detach the one/s limiting your screen size.

$ tmux detach-client -t /dev/pts/2

Detach all but the current session.

$ tmux detach-client -a