temp-buffer-max-height

temp-buffer-max-height is a variable defined in `help.el'.
Its value is shown below.

Documentation:
Maximum height of a window displaying a temporary buffer.
This is effective only when Temp Buffer Resize mode is enabled.
The value is the maximum height (in lines) which
`resize-temp-buffer-window' will give to a window displaying a
temporary buffer. It can also be a function to be called to
choose the height for such a buffer. It gets one argument, the
buffer, and should return a positive integer. At the time the
function is called, the window to be resized is selected.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 24.3 of Emacs.

Value:

(lambda
  (buffer)
  (if
      (eq
       (selected-window)
       (frame-root-window))
      (/
       (x-display-pixel-height)
       (frame-char-height)
       2)
    (/
     (-
      (frame-height)
      2)
     2)))