If you wish to have an overviedw map, you have to click on the corresponding checkbox in the frame "General elements" (on the "Layout" index tab). With this, a dialog opens where the options of the overview map can be set.
If you have several themes activated, you can set the overview map parameters for each theme separately after selecting it from “data frames”.
The size of the overview map can be adapted with the entry boxes “Width” and “Height”. The width-to-height ratio is kept up automatically.
In the list “Layer” you can select the levels to be displayed in the outline map. If there are many layers, it is often easier to deactivate all checkboxes and then select the few which are actually appropriate in such a small scale.
Finally, you can bring the preview up-to-date via “Update preview”. The settings can be saved and reloaded in other projects via the settings file.