An average sized message is around 30 KB.
We said earlier that your mailbox size is determined by the size of its folders. Logically enough, the folder size is determined by the sizes of the messages (or calendar appointments, contact information, notes, etc.) inside it.
Average sizes for messages will vary from organization to organization and from person to person. A very rough estimate is that an average sized message is about 30 KB. And, like mailbox size, message size limitations are set by a system administrator at the server level—they're not something you can change in Outlook.
Most organizations impose some sort of size limitation both for incoming and outgoing e-mail messages. For example, in some organizations, a message over 1 MB may be unsendable or unreceivable. You may want to do a little research and see if your organization or ISP (Internet service provider) has specific size limits for sending or receiving messages.