org-agenda-window-frame-fractions

org-agenda-window-frame-fractions is a variable defined in `org-agenda.el'.
Its value is

(0.5 . 0.75)

Documentation:
The min and max height of the agenda window as a fraction of frame height.
The value of the variable is a cons cell with two numbers between 0 and 1.
It only matters if `org-agenda-window-setup' is `reorganize-frame'.

You can customize this variable.