Changing font size

What is the correct way to change the font size (maybe using the style.css) ?

For example, I wanted to change the font of <p>.
With

.body p{
 font-size: 13px;
}

It didn’t work. I tried placing it in style.css and in css/main.css

I succeeded by adding the font-size in .body{} in css/main.css but this doesn’t sound right.

Also, is it possible to change the font-size of random plugins?