from here: https://www.lifewire.com/increase-outlook-attachment-size-limit-1173679 

Increase the Outlook Attachment Size Limit

To change the size Outlook allows as a maximum for attachments to send:

1. Press the keyboard shortcut Windows-R.
2. Type "regedit" in the Run dialog.
3. Click OK.
4. Travel down the registry tree to the entry corresponding to your Outlook version:
   • 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.
5. Double-click the MaximumAttachmentSize value.
   • If you cannot see MaximumAttachmentSize:
     1. Select Edit | New | DWORD Value from the menu.
     2. Enter "MaximumAttachmentSize" (not including the quotation marks).
     3. Press Enter.
     4. Now double-click the MaximumAttachmentSize value you just created.

1. Enter the desired attachment size limit in KB under Value Data:.
   • To set a size limit of 25MB, for example, enter "25600."
   • The default value (with MaximumAttachmentSize not present) is 20MB or 20480.
   • For no attachment file size limit, enter "0." Practically all mail servers have a size limit, though, so "0" is not recommended; you would invariably get large messages back as undeliverable after an often long and fruitless uploading process.
   • Ideally, the limit corresponds to your mail server's limit. Reduce the Outlook limit by some 500KB to allow wiggle room.

1. Click OK.