How can I change the font size on the post of WordPress?

Question

I have found three websites which address WordPress style sheets:
Playing With Fonts: http://codex.wordpress.org/Playing_With_…
Font Size and Color: http://en.support.wordpress.com/editors/…
How to Change Font Size In Wordpress: http://www.ehow.com/how_5469172_change-f…
Here are the instructions from the third website above:
Things You'll Need:
Site designed in Wordpress
Administrator access
Step 1
First, to change the font size on a Wordpress site you need to log into the site as administrator. If you do not know how to reach the login page, simply add "/wp-login.php" after the "www.url.com" address. Then login using the admin sign in.
Step 2
Next, get to the Wordpress file editor by going to Appearance->Editor.
Step 3
Once in Editor, identify the Stylesheet file (style.css) and click to edit. This is usually located on the right hand side under the "Styles" heading.
Step 4
Once in the style.css file, you want to conduct a search for "fontsize". This parameter is used throughout the style.css file and is the parameter used to control font size. Next to "fontsize" you will see something like "16px". Simply increase or decrease the "16" to change font size. The tricky part is to figure out which "fontsize" you want to edit.
The way to do this is to first look at the heading it is under. If it is under "comment-template", it is obviously related to either the font size of the comments themselves or perhaps the comment form or the date/time stamp of the comment.
It may take a little trial and error to find the right "fontsize" to modify. Simply modify the one you think is correct and in a separate window, reload the page with the font you are looking to modify. If it did not change, undo the previous edit and try a different "fontsize".
I went to the Dashboard and clicked on the “Appearance” icon on the left-hand side, and then clicked on editor. I clicked on Stylesheet (style.css), but I have not been able to find the “fontsize” that would change my post font size. As the author writes: "The tricky part is to figure out which "fontsize" you want to edit." Some of the font sizes are given as a percentage, and I don’t know if these come into question, i.e., if I should try changing one of these to see what happens.
The end result is that I still have not been able to change my post font size, other than by using the Heading-1-through-6 option, which, I understand, is not the best way to do it.

Answer 1

From John Waller: “If you're using IE8, hit F12 to launch Developer Tools which give you a point and click method of locating the relevant CSS rule on screen.”
I have Internet Explorer 8 (IE8) so tried this method.
This sounded like another promising possibility; however, I have not been able to change the font size in my post using this method.
I have gone to Dashboard > Posts > Edit > then moused-over my post > clicked on "edit" and then hit F12. An enormous amount (20 pages or more) of CSS appears. I tried picking out the font size that I thought pertains to the post (under Text Area), and I have changed the font size from 13px to 24px; however, I have not been able to “save” the new setting. When I go back to it, it is back to 13px.
As a result, I have not been able to use a trial-and-error method to establish which font size I must change in order to change the font size of my post.