Can the field length in an attribute table be modified?
No. Once the length of a text field has been defined, it cannot be changed. However, users can create a new field of type 'text' with a maximum length of 255 characters, and the properties of this new field can be changed to mirror the properties of the original field using the Field Calculator.
In an attribute table, each field has properties that describe its contents and how the attributes are shown in ArcMap, and these properties are set when the table or feature class is created. Some field properties can be amended after the table or feature class is created. ArcGIS system properties are listed under the Field Details section, which includes the length of the field, and this value cannot be changed. The default length of a text type field is 50 characters and the field name cannot be longer than 255 characters.
For further information on how to create a text field with an increased character length, please refer to KB article 42783.