Hi Srini,
1. You need a new Z-field, so that you can select the required data type and length based on your requirements.
2. Instead of trying a validation, the preferred option is to have a Z-check table for the values, so that user can select from the drop-down list only.
3. You will also need a Z-transaction to maintain values in that Z-table, you can assign access to this Z-transaction only to master data administrator role to have additional control.
Regards,
Paulo