First you click on plus button to add new server entry, then you dbl-click and fill out name, url and api key columns. Once everything is filled out, you select that server and click on 'Check server' button. If everything checks out, you'll be able to click on 4th column 'Active' to make it appear in newznab tab. After that you click on save and check out newznab tab.
You can have multiple newznab servers in newznab tab.
Regarding RSS size column, there isn't one and you can't add it for now.