Mail servers may reject emails that cross a certain size threshold due to mail attachments that are included in the email. The threshold is usually in the 20 Megabyte region but it is defined by the mail provider, which means that it may be more or less in your case. The problem here is that this is a custom variable, which means that you may run into returned email problems because the receiving email provider has a lower size threshold than yours.
Google some days ago announced that it would add the option to use Google Drive when you are sending emails with attachments on Gmail. The feature has just been enabled on Martin Brinkmann's account and he’d like to walk you through the process. Note that he's using the new compose window and not the old one. To start click on the compose button at the upper right corner of the screen. The new compose window should come up. The bottom bar that begins with the send button lists the attachment icon. Hover over it to see available options.