First you would need to add the field [length] to your table.  Then you
woudl need to make sure that the new field is available to your form.  If
you form's RecordSource property is just the name of your table then the new
field should be available as soon as you modify the table.  If the
RecordSource property is a saved query or SQL Statement you might need to
modify it since the new table field might not automatically be added.

For example.  If your RecordSource is a SQL statement like...

SELECT * FROM TableName

...then the new field will automatically be picked up because the " * "
tells the statement to include all fields from the table. If the SQL
statement lists fields by name...

SELECT [order Id], [order no], [product id], [quantity], [rate], [discount]
FROM TableName

...then the new field would have to be added to the statement before you can
use it in your form.

Once the new field is available in your form you just add a new TextBox that
uses the new field as its ControlSource and then add the name of the field
to your existing expression.