Setting the font size

You can download here the document from this video to practice it by yourself!

What size of text should you use?

Well, the standard for writing paragraphs is 12 points, which is quite good for most cases.
Headings and titles are usually increased to 14 or 16, depending on their importance.

Note that font sizes are usually selected by even numbers, but if you wish you can also select odd numbers such as 11, 13, 15 and even halves, for example 11.5.
These sizes might not appear in the drop-down menu, but you can manually type them down in the font-size box, and then hit the "Enter" key.

Styling the title page of your document (the first page, where you state your name, the job's main title etc.) is where you can put into action the really large fonts such as 30, 40, and even 50 (do some trial and error to see their effect on the page).

While you can quite freely increase fonts, try to avoid using a font smaller than 11, or make sure your glasses are at reach (and try even harder to avoid fonts smaller than 10, or make sure your microscope is at reach).

Fonts smaller than 11 are usually accepted when writing footnotes or comments (or if you are really in a need to save space on your page and squeeze more words into the lines).

Note that even if you manage to make small fonts look fine on the screen, printing them out on a paper sheet might remind you that sometimes what you see on the screen and what is eventually printed on the paper can be two different stories...