Choose the View > Fit in Window command or click the corresponding option from the Quick Options bar at the bottom of the active window.
Another shortcut: Double-click the middle mouse button to Fit in Window.
If the current view has been rotated, the rotation remains intact.
See Set Orientation.
If you wish, you can save a window’s Fit in Window setting as part of any view’s View Settings: choose Fit in Window from the Zooming pop-up in the 2D/3D Documents Panel of View Settings.