Change the zoom level of your database diagram to control how much of the diagram you can see on your screen at once.
To zoom a database diagram
Note To edit tables, you must set the zoom level to 50% or higher. To create relationships, you must set the zoom level to 25% or higher.
To zoom to a selected object
Tip To center the entire diagram in the window, use Select All to highlight all objects in the database and then choose Selection from the Zoom list on the toolbar. This will zoom your diagram to fit in the current diagram window size.