In tmux, the size of window panes can be adjusted incrementally with the resize-pane command. For instance, to resize a pane in any direction (left, down, up, and right), use one of the following commands

resize-pane -L 10
resize-pane -D 10
resize-pane -U 10
resize-pane -R 10

This will adjust the pane by 10 units in the corresponding direction. Of course, other values can be used for more significant size adjustments.