In DW, if you don't want to change your theme, go to customize style and the tab "Fonts" and you can change font sizes for your journal.

Question about Wordpress, do you type your posts directly in Wordpress or do you write them on Word or something and then paste? Because if it's the latter, you might want to use the HTML side instead of rich text, because rich text will carry over all the code nonsense from Wordpress, which will also define the font in your entry.

Your font size in both also looks pretty standard to me, so check that it isn't just your browser's zoom setting.