Password Length Inspector will help you identifying the input length constraints of ANY password field on any website.
After installing this extension, you will find a small indicator next to every password field, telling you how many characters may be entered into that specific field. In case there is no character limit, infinity will be displayed. This way, you will have never to figure manually (for instance by digging in the source code of the website) how many characters are allowed to be entered in a password field.
As an indicator is displayed right next to the password field, it sometimes can happen that it covers for instance the submit or login button. Fortunately, this extension allows the indicator to be dragged around by the mouse in order to reveal any important controls or other parts of the web pages, being covered by the indicator.