You can use the mouse to zoom in on or out from the artboard. You can also press CTRL+9 to make the selected object fill the entire view so that you can work on it in detail. To quickly zoom back to actual size, press CTRL+1.
You can also quickly move the artboard around by holding down the SPACEBAR while dragging anywhere inside the artboard.
All the zoom and pan options are described in Zoom or pan the artboard.
You can view the visual design of your document in a What You See Is What You Get (WYSIWYG) format in Design view, or you can view the XAML markup that Expression Blend generates for your design in XAML view. You can modify your application in both views, although modifying XAML by hand can cause errors if you are not familiar with XAML syntax. For information about changing the artboard view, see View your design or the XAML that defines your design. For more information about working with XAML, see Editing XAML.
In Expression Blend, you can also view and modify a code-behind file for your document by double-clicking the file name (ending in .cs, .vb, or .js) in the Projects panel.
For more information, see Modify a code-behind file.