I have read in past blogs a bunch of different maximum size attachments that Gmail will allow you to send through their SMTP servers so I wanted to verify directly through Google what the maximum actual attachment size was.
Maximum Gmail Attachment Size:
Currently the max attachment size that Google will allow you to send or receive via Gmail is 25 MB (Megabytes). Please note that the actual attachment size may be a bit smaller as email can add size for the email headers or details that are required to actually send the attachment. It is safe to say that if nothing else is attached to the email that you can easily send 20MB attachments without issue.
Also the above is true currently on the date of this article which is 08/03/2009. Google has raised the maximum file size numerous times in the past and it would not suprise me if they raised it again so if you want to check the current maximum attachment size that can be sent through or received by Google’s Gmail SMTP servers check this link.