There are two navigator views. The one that appears when you select "Navigator show/Hide" from the Track view menu or the one that appears in the multidock when using the main menu view or pressing Alt+Shift+8.
If it is the former it can be resized by click dragging on the divider line between it and the track view. If it is the later it responds in exactly the same way as any other dockable view does.
Are you seeing something different to either of those?