To modify the default attachment limit size in Outlook for an Internet email account yourself, follow these steps: 

1. Exit Outlook.
2. Start Registry Editor. (run regedit.exe)
3. Locate and then select one of the following registry subkeys:
    
   HKEY_CURRENT_USERSoftwareMicrosoftOfficex.0OutlookPreferences
   HKEY_CURRENT_USERSoftwarePoliciesMicrosoftOfficex.0OutlookPreferences
   Notes
   • The x.0 placeholder represents your version of Office (16.0 = Office 2016, 15.0 = Office 2013).
   • Manually create the path in the registry if it does not currently exist.
4. Add the following registry data under this subkey:
    
   Value type: DWORD
   Value name: MaximumAttachmentSize
   Value data: An integer that specifies the total maximum allowable attachment size. For example, specify 30720 (Decimal) to configure a 30-MB limit. 값을 0으로 하면, 무제한용량.
   Notes
   • Specify a value of zero (0) if you want to configure no limit for attachments.
   • Specify a value that is less than 20 MB if you want to configure a limit that is less than the default 20 MB.
5. Exit Registry Editor
6. Start Outlook.