Text Font Size

How do I change the font size in the body content for all my web pages without effecting all the other menu tabs and modules? Right now it's 11px, it's too small and I need to change it to 13px. Can you tell me what file to change?

Thanks,
Karen

Well probably

.body p{
...
}

will affect only articles.

I've tried to look for the CSS file to change the font size but I have no luck yet. It's not located in style.CSS, layout.CSS, template.CSS, nor in index.php. What CSS file do I actually go change to?

The .body p{
...
}

does not have font sixe listed here.

I mean add it any ware.

I've tried to add the font size under body in the layout.CSS but nothing happens.

Give me link