Set the parameters defining the dimension text:

How to use Dim. Text:
1. Select the parameters defining the dimension text.
2. Enter a dimension value if needed.
3. Select the precision of the dimension.
Selecting Textual allows you to enter any text as the dimension text. (Up to 19 characters).
Selecting Manual allows you to enter any dimension. A line appears under the dimension to indicate that it does not reflect the model's dimensions.
See Dimension Parameters for details