I have tried to use the gtk_widget_set_size_request function to make the dialog more compact. Fortunatelly, it works quite well, as you can see. At the moment I only have tried the old (probably deprecated) file dialog (I think that this one is better and more usefull than the new one).
The actual code is (as you may expect):
gtk_widget_set_size_request (dialog, 320, 240);
And that’s all.