The character limit for Alphanumeric Fields is 255 characters. This means; the information that you insert into this field; has a maximum limit of 255 characters. If for example; the value; 20 is inserted in this field during creation; the field will be limited to 20 characters total. Currently fields cannot exceed 255 characters.
Once a field is created; you CANNOT adjust the character limit. Please ensure you account for the maximum length of the information you will be capturing in this field at the time of setup.