Basically, you need to wrap the word in a <span> tag, and apply either a class or an inline style to the span. Using a class is better because you can reuse it in other pages, or other parts of the same page. Also, changing the font size in the class will apply the changes to all instances of the class. With an inline style, you have to change each one individually.
To wrap a word in a <span> tag, select the word, then press Ctrl+T (Cmd+T on a Mac) to bring up the Wrap Tag dialog. Type "sp" (without the quotes), and press Enter twice. Then create a class using the CSS Designer. Apply the class to the word by selecting it again, and select the class from the Class drop-down menu in the Property inspector.
To create an inline style, select the word, and click the CSS button on the left of the Property inspector. Select the font size you want from the Size drop-down menu. If the size you want isn't listed, just type the value in the Size field.