You cannot change the sizes of the fonts on pages and posts by doing what your are doing ie. using the editor. The format in the editor is only for fonts within posts and pages. The fonts for the post and page titles are supplied by the theme you are using. These are the 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 HTML editor. http://en.support.wordpress.com/editors/font-size-color/
(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/
Note: Any font changes to font families, colors and size in the blog title, tagline, post titles, page titles, categories and tags cannot be done without CSS editing.