Go to admin panel-> Post->New Post-> select visual-> Click show/hide kitchen sink->Click Paragraph->select (u want)
I also had that problem, I solved it. You can use <font> or one of the font setting in the Administrator of WordPress. enjoy Jane
I believe you can change "text-area" or "post-area" - I think it's one of those in your CSS stylesheet, but you have to know CSS to be able to do that. I believe the code to add is font-size:15px; THAT would just be an example of course....I hope that helps!
I wrote simple plugin which enable fontsize option in tinyMCE visual editor back. Hope you find it useful! http://wordpress.org/extend/plugins/wp-editor-fontsize/
If you write in an outside editor, like Word for example and paste the text to WP, you will unnecessarily load the HTML code. Paste the text as simple text or write as simple as you can and use the font editor from WP first and then use a WP theme generator with a flexible typography tool on order to make the changes to the text. I use Lubith, and it allows me to change various font properties (even line height and word spacing) to any element of my WP layout that contains text.