When I am typing in a post, I don't see any way of just highlighting a few words and changing the font size only for those few words.
See (1) below.
There is no free and easy way to change fonts in a free hosted WordPress.COM blog. These are 3 ways to change fonts on WordPress.com blogs.
(1) Changing font families, colors and size in text in posts or pages either post by post or page by page by coding into the Text HTML editor. http://wpbtips.wordpress.com/2009/07/11/formatting-text-pt-5/
(2) Changing font colors in text in posts or pages either post by post or page by page using the #4 icon (Select text color – change the text color) in Row 2 the Visual editor. http://en.support.wordpress.com/visual-editor/#row-2
(3) Changing all font families, colors and sizes throughout the whole blog by purchasing an annually renewable Custom Design upgrade and using Typekit Fonts with Staff support. http://en.support.wordpress.com/custom-design/#custom-fonts