Q. How large can attachments be in an email?
Outlook has a default limit of 20MB (20480 KB) in attachments, ie if you go over your email will not be sent.
To send files over 20Mb, instructions on how to first upload these to OneDrive to share can be found here.
To tell whether or not your attachment is over 20MB, check the small icon in the attachment screen, shown below. Or right-click the file and select Get Info to find the file size.