tmux detach-client

With tmux list-client, you can list all clients connected to tmux sessions. For instance:

$ tmux list-client
/dev/pts/6: 0 [25x80 xterm] (utf8)
/dev/pts/8: 0 [25x80 xterm] (utf8)

From this point, you can choose to detach a specified client, or all clients of a specified session. Say I want to detach everyone connected to session 0:

$ tmux detach-client -s 0

Then, you can attach the session so the size will be yours.

Actually, all that can be done with tmux attach -d (the -d option force all other clients to detach).