When a form text field is inserted in a Word document, the grey shaded length is about 5 characters long. How can this length be increased?
The grey shading is just to indicate there is a form field there. By default the field will hold unlimited text, unless you change the field properties to only be a certain length. Still, the shading will only show a few spaces until you type, then the shading grows as you continue type.
As far as I know, there is no way to extend the shading when there is no text in the field. If the field is in a table, you could shade the entire cell.