What is the attachment? A MS doc?
Yes, from an email.
Anyways, just save the attachment to your desktop or something, then right click on it and select Open With -- select TextEdit to view the doc.
What kind of document is it?
.doc, .rtf, ....?
You should see the file extension in your Mail programme or when you click the file on your desktop and Get Info about it (Command + I).
How big is it? And does it fit, what it should contain?
You will need to either have iWork 08 (w/ Numbers) or Office 2008 for Mac