I have not had to do this myself, yet... But, here is the document that should help get you going in the right direction.
Simple answer would be, at this time, there is no out of the box toggle or attributre you can select a minimum length. However, there is a way, using calculations, to determine and prevent progress by using the calculation on a condition or decision.
http://community.landesk.com/support/docs/DOC-26537
Read the entire document for better insight, but what you are looking for towards the bottom. It looks like you can use two different methods:
1. IsNullOrEmpty() method, which looks to see if the attribute is Null or Empty. I don't see their definition of Emtpy, hopefully it includes spaces.
2. .Length method, use <, >, or = to determine length
Hope this helps you. Let me know your outcomes!