Click Navigation on the button bar if the bar is not activated yet.
In the Navigation bar, activate the Model overview tab.
You can move the frame to change the visible area of the model.
The frame indicates which part of the model is visible. The size of the visible area is determined by the appearance size you set. You can set it in various ways:
Use the buttons in the View tab bar.
Press the Ctrl key, and at the same time roll the mouse wheel while the mouse pointer is placed over the modeling area.
Enter a value in the Size of appearance box of the model properties (Format > Representation). The lower the percentage you enter in the Size of appearance box, the larger the visible area and thus the frame of the model overview, and vice versa.