On the Visual Editor on posts and Pages, click the last button on the toolbar (Kitchen Sink) to display a second row of buttons. There are font features there.
For section titles or subtitles, I recommend you use the heading options for H1 – h5 found in the drop down menu which starts with Paragraph. Click it to choose a heading. This is the proper way to create subtitles, not by forcing fonts and font sizes on the text.
About the posts and the pages i understood but what i really want is to change the size of the post’s and page’s font which will be published in the theme menu..have you any idea how to do it without buying the custom design??