Yra,

There is no Developer app > fields

What I see on developer are icons for 5 these choices:

Settings, polyglot, forms, navigation, pages

The version I am using of UNA is

Version: 11.0.3 Installed: 12 Apr 2019

Michael Beloved Yra LeonidS 

Studio > Developer > Forms > Drop Down to System > In System Name column, you will see Comment. Click on 2 displays link under Displays column for Comment. You will see Comment Post Form > Then in Fields Column, click on 7 Fields. You will then see System Name column...scroll down to Text and click the Pencil icon to edit the field. 

Change Check for from Data Length to Data Availability. It will by pass checking character count and only checks to make sure that at least 1 character was entered and cannot be blank. It will no longer check to make sure you are not over 5000 characters. You will not need to do any sql commands for this to work so don't worry about running the commands you were told to use.

See attached image