If you want a wider dropdown, you can also increase your padding in order to push the text below or to the right of the box.

No Padding:

<ComboBox Name="BlankBox"
                              Height="21"
                              Margin="3,2"
                              Padding="0"/>

(text is aligned top-left)

Some Padding:

<ComboBox Name="BlankBox"
                              Height="21"
                              Margin="3,2"
                              Padding="5"/>

(text is offset from the top-left and partially obscured by the bottom padding of the box)

Full Padding:

<ComboBox Name="BlankBox"
                              Height="21"
                              Margin="3,2"
                              Padding="20"/>

(text is fully out of view)