GtkWindow::set_default_size

set_default_size() sets the default opening size of a window, in pixels. This can be overridden by size requests from the window's child widget exceeding the default size of the window, or by actively setting the resizing policy of the window so that the window always shrink-wraps its child.

Note that a smaller child widget whose size has not been set or otherwise restricted by the programmer will generally expand to fit the default size of the parent window.