Go to Regedit Go to Outlook 2010: HKEY_CURRENT_USER\Software\Microsoft\Office\14.0\Outlook\Preferences. Outlook 2013: HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Outlook\Preferences. Outlook 2016: HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Preferences. click Edit > New > DWORD (32-bit) Value then rename the value to "MaximumAttachMentSize." Add a size limit (in kilobytes). 0 is no limit.