Printing in A0 format

Hi everyone. I use Draft Sight on MAC OS (language Italian) and I have problems with the setting of the paper format. Cause the format A0 is not in the default list of formats I have to add it manually. Anyway the option "manage paper dimensions" (Gestisci dimensioni) turns to be deactivated. So I cannot set A0 or even A3! 

Can somebody help me?