Alerting the user of a large size file when using Send in Word add-in

The 'Edit and Send' option in the FT button in Salesforce allows you to do just that - edit the document and then send it out.
When using this option on a Word document - it downloads and opens in the Word application, where it can be edited and then sent 
using the Word add-in "Email" option.
Thing is, there is a limit to the file size that can be sent from Word (3.5 MB) which made the file arrive empty.
Seeing as this limit is not in our hands, we've decided to alert the user when he reaches it so that he can fix the document. 


Enjoy!