Use the Zoom tool to change the viewable area in the window, and the Thin Lines tool to maintain the true thickness of lines relative to the zoom of the view.
Zoom in a view using the navigation bar:
If the navigation bar is hidden in your view, click View tabWindows panel
User Interface drop-down
Navigation Bar.
You can also zoom project views using SteeringWheels.
When you zoom in on a model in a small scale view, element lines may appear thicker than usual. To improve their appearance, use thin lines: click View tabGraphics panel
Thin Lines.
This tool affects all views but does not affect printing or print preview.