If your internet connection isn't strong enough it can be hard to open large email attachments. Simply the bandwith is not large enough. If you can afford it I suggest upgrading to a better connection.
Earlier version of Outlook limit the size of file to 2gb. In order to view this file you must use another program. Microsoft offer a program that will remove part of the file so it can be read but part of the file will be un-viewable. A program called Advanced Outlook Repair claims to do the job also.