Ah, okay… the screenshot you are showing in your initial post is not an input (it looks like a regular text element), but if you really are using an input with a Content format of Text, then you can set a maximum number of characters like this…
If it helps, the red box at the top of the following screenshot is a text element (which is used to display text as opposed to a user being able to input text into it), and the red box at the bottom is the input that will give you the maximum characters option shown in the screenshot above…
Best…
Mike