How to set max length in textfield

How I can set a maximum length number for a text field? Like it is shown in the text field in private messages, so best would be, to have the number automatically actualized by typing in.

ah, I found it in the field area in the backend, but would it be possible to have a counter as in the private message text field?

Maximum Length can be set by editing the field in CB > Field Management under the Global tab.

but would it be possible to have a counter as in the private message text field?

There's no character counter for fields. You'd have to add that your self likely using custom JS and the layout parameters. Shouldn't really be necessary as the input will stop accepting characters and the validation will prevent going over the limit.