resize-mini-windows

resize-mini-windows is a variable defined in `C source code'.
Its value is

grow-only

Documentation:
How to resize mini-windows (the minibuffer and the echo area).
A value of nil means don't automatically resize mini-windows.
A value of t means resize them to fit the text displayed in them.
A value of `grow-only', the default, means let mini-windows grow only;
they return to their normal size when the minibuffer is closed, or the
echo area becomes empty.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 25.1 of Emacs.