We do not have a set limit on the message size for inbound or outbound email. There are, however, practical limits.
If you send a very large message (larger than a megabyte), you will want to check with the recipient to make sure they are setup to receive larger messages.
Likewise, inbound messages larger than a megabyte may also take a very long time to download or may fail to download all together. Your ability to receive larger messages will depending on the speed of your Internet connection and the configuration of your email software.