Preferably less than 2MB. E-mail system will have problems when dealing with large attachments, and sending large attachments will also affect the speed of sending and receiving emails. To avoid this situation, you can try to compress the attachment with compression software before sending it to reduce the file size. Introduce yourself briefly in the body of the email, and attach a resume summary and contact information, so that even if the attachment can't be opened, the other party can understand your basic situation.