ediff-make-wide-display-function is a variable defined in `ediff-wind.el'.
Its value is

  • Automatically becomes permanently buffer-local when set.
  • This variable may be risky if used as a file-local variable.

The value is a function that is called to create a wide display.
The function is called without arguments. It should resize the frame in
which buffers A, B, and C are to be displayed, and it should save the old
frame parameters in `ediff-wide-display-orig-parameters'.
The variable `ediff-wide-display-frame' should be set to contain
the frame used for the wide display.