What is preferred spacing in a document?

It really depends on the recipient.

But during my career the most used were single or 1,5 line spacing.

If you need to know how to space in MS Word, click on the related link bellow.

Another Answer

It's a good idea to review the style guide in use by the publisher, professor, reader, reviewer to determine many format criteria for your document.

Originally, a typesetter trained to set type knew -- depending on the type face -- how long to make a line (how wide to set the margins), how much white space to leave between lines (the spacing question, above), all based on the parameters recommended for the human eye to be able to finish one line of text and immediately find the beginning of the next line.

To mimic this skill, set your font size, margins and line spacing on the page so that your document is easy to read. Be consistent about where you place page numbers, foot notes and so forth.

For example, single spaced, Times Roman font set at 8 points with half-inch margins left and right might allow you to cram more text onto a page, but this setup is not easy to read.