In the family types dialogue click on Manage Lookup Tables, then in the next dialogue select the table named and click to export it.

This will create a CSV file. Save it in a temporary location (you can delete it later).
Open the CSV file using a spreadsheet app.
You'll see something like this:

Add a row to the table and populate the values for the 20mm size fitting:

Save the file in CSV format.
Go back to Revit and in the Manage Lookup Tables dialogue import the revised CSV file and overwrite the previous version

20mm size fitting should now work.
As I said earlier though there is maybe a reason why there is not already a 20mm size included so it might be that the manufacturer does not sell that size?