Thank you for contacting Aspose support team.
You can use following sample code to get the mail box size (in bytes) which is utilized. Subtract this number from your mail box quota assigned by the mail server administrator. It will return you the free mail box size. Please give it a try and share the feedback.
IEWSClient client = EWSClient.GetEWSClient("https://outlook.office365.com/ews/exchange.asmx", "user", "password");
long iSize = client.GetMailboxSize();