From the component browser in AppDesigner, select the figure handle and check the Window Appearance & Position properties in the properties inspector.
If the WindowState is set to maximized or fullscreen then the AutoResizeChildren should be checked so that different the app components will maintain their relative positions with monitor sizes.
You also need to make these changes in the startup function. This will allow the app to fully render before resizing the figure.
app.System_Inert.Items = app.Material_list;
app.System_Reactive.Items = app.Material_list;
app.System_Reactive.Value = 'Water';