You can create a condition on that table.
Expand your datastore in the model and right click on Constraints --> New Condition.
On the name field, type something like "Check the length isn't too long"
and on the where clause set LENGTH(AB_ART.LIBART) <= 28
.
You can also add a message that will be included in the Error table.
As the error table is created based on the target table, the field will be too short as well. So you will have to edit the length of that column in your datastore.
Then just turn on Flow Control in your interface and enable that condition on the Control tab.