Font size is a very complex subject. Later you will learn some of that but for now keep it simple.
The easiest way to set size is with pixels. Add a style attribute to the fourth list item in textcss.html. Add this as the value for that attribute.
font-size:12px;
There are some better ways but this one does work and is very simple. Stick with it for now. You will learn the alternatives later. Save and view and the fourth item should be smaller.
The default font size is usually 16px. Headings will default to being bigger but you can style p, body, h1, h2, li etc. as you want. Just add new type rule sets to your style sheet.