That will probably depend on the version. In Word 2003, I give the command "Insert - Table". There is also a button in the toolbar, that looks like a table - you can click on it.
first select home tab in word 2007/2010 place your insertion point inside word you want to be header (simply click inside that word)and on home tab look at right side
The number of megabytes on a word document depends on how much text, pictures, or other objects are in the document. Images take up more space then the text.