angular.module('myApp',['ngMessages']);

<script src="//ajax.googleapis.com/ajax/libs/angularjs/1.3.2/angular.min.js"></script>
<script src="//ajax.googleapis.com/ajax/libs/angularjs/1.3.2/angular-messages.js"></script>

<form name="form" ng-app="myApp">
  <div ng-init="minlength = 5">
    <input type="text"
      name="name"
      ng-model="name"
      ng-minlength="minlength"
    >
    <div ng-messages="form.name.$error">
      <div ng-message="minlength">
        Your field must be at least {{minlength}} characters long. 
        Current length: {{form.name.$viewValue.length}}
      </div>
    </div>
  </div>
</form>