That depends on the attachment. With Gmail, you can send and receive messages up to 10 megabytes (MB) in size.
When you add an attachment, the size of a file may increase because transport encodings (the information that allows your message to be safely sent and read) are automatically added.
This means that in some cases, attachments that are 6 to 10MB in size may push the total message size above 10MB. When this happens, Gmail displays a warning that your message exceeds the 10MB limit.