“The file you’re attaching is bigger than the server allows.” error message will be shown when a user tries to attach a file which is over 20 MB. As suggested in the message the best solution is to copy the file to a shared location and send the link to the recipients. Another alternative is to archive the file and see if it can be reduced below 20 MB (20 MB is default attachment size limit.)
But if you still want go ahead and still attach the file then you can try out the below registry hack. Please note that this hack only for the client side and all mail servers would still have rules to check file size limit.
Launch Registry on Windows 8
For this demo, the registry changes are done on Windows 8 system. Right click on Start button, select Run from the menu.
In Run window type Registry and click OK button to open Registry.
Windows Registry changes needs to be done with caution. Do it at your own risk.
Registry changes on Windows 8 for Outlook 2013 attachment limit
In the Registry application, navigate to COMPUTER –> HKEY_CURRENT_USER –> Software –> Microsoft –> Office –> 15.0 –> Outlook –> Preferences. (HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Outlook\Preferences).
Right click on Preferences and select New then DWORD (32-bit and 64-bit based on your operating system installation).
Enter the name of the key as MaximumAttachmentSize. Then double click the newly added key and add Decimal Value as 25600 (if you want to increase the limit to 25 MB (25 * 1024)).
Click OK button to confirm and apply the changes. This registry hack should allow you to insert attachment file size up to 25 MB in Outlook 2013.