If you want to restrict the number of characters in a text field, please read https://www.quickbase.com/help/set_max_length.html.
If you want to limit the length of another field type, the only workaround is to create a separate formula text field that uses the Length() function and perhaps build a view of non-compliant records. More details about the Length() function can be reviewed here: https://www.quickbase.com/db/6ewwzuuj?a=dr&r=b4&rl=e3qq