Truncate username while capturing in transition action

If this variable is populated in datacard you can use dispatch set variable in combination with left formula to get the no of chars you need. Dispatch action should be triggered with change of state.

You could build a SQL query to populate a List that has them automatically truncated and then offer that list on the data card. The Dispatch option would work as well. Either way, you'll have to have a variable to hold that truncated value and then maybe another variable with the proper full name value.

Another option is to use a List with Alias