Service Portal - How to truncate a user selection string field

Good Moring!

I am trying to prevent a user from typing more than 20 characters into a string field.

In the past I would have put a validation rule on the filed and just prevented the requester from submitting the request if there were too many characters

In this case this field is going to be used as part of a concat and in need to truncate the field live.

Is there a way to do this?

Thank You,
Chris Lawrence