You want to limit the length of the string input in a text field.
Set the text field's maxChars property.
By default, an input text field allows a user to type in as many characters as she desires. However, you may have good reason to want to set a maximum. For example, if an input field prompts a user for her two-character country code, you might want to prevent the user from entering more than two characters. Setting the maxChars property to a number limits the user input to that many characters.
myTextField.maxChars = 6; // Maximum of six characters can be input
Set maxChars to null to allow an entry of unlimited length.