if you change the font size of the header, then the change is proportional to the size of icons and the titlebar itself.
css file:
.custom-window-titlebar {
-fx-font: Tahoma;
-fx-font-size: 60px;
-fx-fill: rgba(255,255,255,50);
-fx-alignment: center;
}
and set style class:
Window window = new Window("My MDI Window");
window.setPrefSize(1760, 770);
window.setTitleBarStyleClass("custom-window-titlebar");