Answer: The word that is double-clicked will be selected.
This is useful if you want to copy-paste a certain word. For example, if you want to search out its definition, synonyms, or example usages in a sentence. Another use is to select the entire word to delete it or replace it with something else (it's handy if the word is quite large).
Double-clicking on a word also works in web browsers and other programs. Triple-clicking will select an entire paragraph of text.
If you want to increase font size, then select the word(s) you wish to enlarge. Then hold down the control key, and press the ] key. This key is located around P. However, some keyboards may have different layouts.