Go to the cell and hit "ctrl-b" then ":" to type the following

:resize-pane -D (Resizes the current pane down)
:resize-pane -U (Resizes the current pane upward)
:resize-pane -L (Resizes the current pane left)
:resize-pane -R (Resizes the current pane right)
:resize-pane -D 10 (Resizes the current pane down by 10 cells)
:resize-pane -U 10 (Resizes the current pane upward by 10 cells)
:resize-pane -L 10 (Resizes the current pane left by 10 cells)
:resize-pane -R 10 (Resizes the current pane right by 10 cells)