You can send up to 25 MB in attachments. If you have more than one attachment, they cannot add up to more than 25 MB. 

If your file is greater than 25 MB, Gmail automatically adds a Google Drive link in the email instead of including it as an attachment. 

For more detailed instructions, visit the Google Help article on how to send attachments with your Gmail message.

When you attach a Google Drive file to a message, Gmail checks to see if your recipients have access to the file. If they don't, you'll be prompted to change the sharing settings of the file before you send your message.

For more detailed instructions, please refer to the Google Help article on sharing settings for Google Drive files.