from : http://superuser.com/questions/456775/tmux-auto-resize-panes

I suggest resizing multiple panes with one of the five tmux presets (see tmux manpage):

C-b M-1             # vertical split, all panes same width
C-b M-2             # horizontal split, all panes same height
C-b M-3             # horizontal split, main pane on top,
                      other panes on bottom, vertically split, all same width
C-b M-4             # vertical split, main pane left,
                      other panes right, horizontally split, all same height
C-b M-5             # tile, new panes on bottom, same height before same width

M denotes the meta key, usually bound to ALT.

On Macs the meta key is usually Esc