The "Basics of Validation Rules" section in the "Creating Validation Rules" help page reads as :

Basics of Validation Rules 

1. A validation rule for a field can consist of conditions like whether a value needs to be filled in, a specific expression it shouldn't contain, the minimum and maximum length of the data, whether it needs to start or end with something, or if it shouldn't be empty. 

But when I select a text field and try to use the conditions on it then I'm presented only with the condition filters of :

There isn't a filter to define the length of the text string. I want to enforce the agents to enter a string with certain length limits.

Also a field can't be used twice in a condition expression. What I mean by that is this :

I'm trying to say to the system that if a text field is filled in with a value then apply the text string length rule. For example I don't want to enforce the agents to fill in a serial number for a ticket but if it has been entered then use these length limits for it.

As a side note I want to express that the filter types for certain data types should be listed on the help guide. I had to find them by trial.