The body font size in TwentyEleven is listed on line 316 in style.css
body, input, textarea {
color: #373737;
font: 15px "Helvetica Neue", Helvetica, Arial, sans-serif;
font-weight: 300;
line-height: 1.625;
}
Copy it over to your child theme style sheet and change the 15px to 20px.
If you have already done this and it’s not working make sure your child theme style.css has the correct headers:
/*
Theme Name: Twenty Eleven Child
Description: Child theme for the Twenty Eleven theme
Author: Your name here
Template: twentyeleven
*/
@import url("../twentyeleven/style.css");
/* Add your over ride rules below this line */
If you need to debug your stylesheet and see which rules are being used by the browser you can use the webkit inspector included with Chrome or the Firebug Firefox plugin.
To use the Chrome inspector highlight a section to inspect then right click and choose inspect element.
You can also make on the fly css changes to see what they will look like. (Changes won’t be saved when the page is refreshed).
See Screenshot Below:
