Margins and lengths of lines of text
It is easier to read a line of text that contains 50-70 characters and spaces.
As a general rule, a margin of at least 2cm around the text is preferable. This is flexible, depending on the type of document. Some letterheads feature a wide left margin, and text looks neater lined up with this.
Remember that photocopiers and fax machines cannot print in the small margin around the edge of a document, and the priority is always for text to be easily read and copied. You should print and copy double-sided wherever possible. Matching margins on left/right and top/bottom makes this look neater.
Do not make margins so large that very little text appears on a page. This can use more paper than necessary and make a document appear longer.