Also, how can you change the font size of the posts texts alone?

I see you've got this one covered as well using this custom CSS:

.entry-content {
	font-size: 113%;
}

Double nice work!

How do you change the comments size by using CSS?

I checked one of your recent posts http://thegoonermonkey.wordpress.com/2013/06/10/monday-news-round-up/ and I see that you already figured this out by using the following custom CSS:

ol.commentlist {
	font-size: 14.5px;
}

Nice work!

Am trying to change the Custom Font size of the Responsive theme but it doesn't seem to be working.

It sounds like perhaps your CSS either didn't have the right selectors or wasn't specific enough.

Here is an example to get you started. This will change the font size for post titles on the home page as well as singel posts and pages in the Responsive theme:

h1.post-title,
h1.post-title a {
	font-size: 30px;
}

Adjust the 30px value as needed.

Note that you can also right-click on elements on the page, select the "Inspect Element" option, and that will open a panel where you can see all the CSS that is being applied to what you clicked on. You can usually find the required CSS selector by reading through the rules listed in the panel on the right.

My method would require that you do this for each media file separately. The Custom Design upgrade would allow you to change your theme's stylesheet to make this a global change for all media files (where you've added a caption).

You can make the gallery caption text larger and bold with CSS, which requires the Custom Design upgrade. I've included the code you would need below, and you can go to appearance > custom design and then click on the "Preview CSS" button in the CSS module at right. Paste the following code in and then you can adjust the font size as desired and on your site. Just navigate to your Photo Gallery post.

.slideshow-slide-caption {
font-size: 13px;
font-weight: bold;
}

Let us know if you have any questions.

No changes you try out aside from those such as Custom Background, Custom Header and Theme Options which are stated on the theme description pages for each theme will be visible to visitors unless you purchase an annually renewable custom design upgrade and do the required CSS editing.
http://en.support.wordpress.com/custom-design/
http://en.support.wordpress.com/custom-design/editing-css/
http://en.support.wordpress.com/custom-design/#frequently-asked-questions

Would someone be able to help me with the CSS code I need to input to change the "fanwood" typekit font to a larger size for:

A) The Header Text
B) The Tagline Text
C) The Body Text
D) The Caption Text

Thank you in advance!

@dogsbuddy:
a) Code tags are used to display computer code, not HTML or CSS. To display code on this forum, you enclose it in backticks (see the note on "Allowed markup").
b) Turn your CSS code to this:

#site-description {
    font-weight: bold;
    padding-top: 100px;
    width: 220px;
}

And add this as well:

.wf-active #site-description {
    font-size: 120%;
}

#site-description{font-size:120%; font-weight:bold; width:220px;}

I have both the Custom Design and CSS upgrades but the Responsive theme still doesn't seem to be responding to my changes ;-)

These are 3 ways to change fonts on wordpress.com blogs.
(1) Changing font families, colors and size in text in posts or pages either post by post or page by page by coding into the HTML editor.
http://wpbtips.wordpress.com/2009/07/11/formatting-text-pt-5/

(2) Changing font colors in text in posts or pages either post by post or page by page using the #4 icon (Select text color – change the text color) in Row 2 the Visual editor. http://en.support.wordpress.com/visual-editor/#row-2

(3) Changing all font families, colors and sizes throughout the whole blog by purchasing an annually renewable Custom Design upgrade and using Typekit Fonts.
http://en.support.wordpress.com/custom-design/custom-css/
Custom Design - Frequently Asked Questions
http://en.support.wordpress.com/custom-design/#frequently-asked-questions
http://en.support.wordpress.com/custom-design/editing-css/
http://en.support.wordpress.com/custom-design/custom-fonts/

Am trying to change the Custom Font size of the Responsive theme but it doesn't seem to be working. The header titles remain their default large size. I'm using Firefox 21.0 under Linux Mint 14.

Sincerely, Phil

Thank you very much to anyone who could help!
http://dogs-buddy.co.uk/

Thanks again for your first response!

Unfortunately, the vertical spacing (leading?) between each row remained large, so now there is too much blank space between each line of text. Is there another bit of css that could address that?

Thanks very much for your time by the way.

.entry-content {
	font-size: 80% !important;
}

To make the base font in the Ryu theme smaller, you could start with something like this:

body, button, input, select, textarea {
     font-size: 16px;
}

Any other elements that are set by the theme elsewhere would also need to be overridden using custom CSS. For example, here's how to reset the main title sizes to make it 2 times the size of the base font size:

.entry-title {
	font-size: 2em;
}

a) If you mean a little to the right, add this to .site-info:before:
margin-right: 14px;
Change the value to adjust the space.
I would also suggest centering the whole thing; try adding this to .site-info:
text-align: center;

b) If you mean copyright notice left, credits right, add this to .site-info:
text-align: right;
and this to .site-info:before:
float: left;

if I 'Delete the font-size rule from .site-info:before,' it deletes my footer and keeps just the WP footer.

I don't think so! When you say .site-info:before: content such-and-such, you say display this content before the credits. It should show up no matter if you specify a font size or not. The font size you apply to .site-info will apply to the extra content too, so specifying a separate font size for it is redundant unless you want a different font size. Perhaps you made a typo. It should be like that:

.site-info {
    font-size: 70%;
}
.site-info:before {
    content: "Copyright © 2006-2013 summerhibiscus.net All Rights Reserved.";
}

If there's a code to put the WP footer above my footer, that would be great.

You would do it as you originally did it (display block), only with "after" instead of "before":

.site-info:after {
    content: "Copyright © 2006-2013 summerhibiscus.net All Rights Reserved.";
display: block;
}

But I think it looks better on one line.

a) As I said, you must delete this from .site-info:before:
font-size: 12px;
b) Did you turn everything to one line by accident or on purpose?