Kindly forgive my ignorance but I am unable to see how this will give two font sizes in a running text. I shall be grateful to know the exact procedure.
As I described above, here are the two methods:
Text Editor Method
Click the Text editor button to change modes. Put span tags with some font information around the paragraph whose size you want to change. Example:
This is some <span class="style=font-size: 14px">larger</span> text.
Feel free to try this now and let me know how it goes.
CSS Method
If you purchase the Custom Design upgrade, you could define a style in your CSS tab and then apply it to your text anywhere you like. For example, to define a class called "larger" that is 14 pixels, you would put this in your CSS area:
.larger {<br />
font-size: 14px;<br />
}
Then in your post, add the class to your span with tags, like this:
This is some <span class="larger">larger</span> text.
The "larger" style you've created in your CSS will be applied automatically to the text around which you've added the span tags.
Some CSS Basics
http://en.support.wordpress.com/custom-design/css-basics/
As notawoodpecker mentioned, another option would be to use a separate editor intended for coding websites, and then paste the generated code into the Text editor in WordPress.