To change WordPress Twenty Twelve layout, first of all you need to

• Login your WordPress administration.
• Go to Styles
• Select Stylesheet (Style.css)

How to change the text larger?

To change the text of WordPress Twenty Twelve theme larger,

• Go to the section /* Body, links, basics */ and then at body { …
  body {
font-size: 14px;
font-size: 1rem;
font-family: Helvetica, Arial, sans-serif;
text-rendering: optimizeLegibility;
color: #444;
}
• Make a change at font-size to the size you want.
• Example:
  At the default,
  font-size: 14 px; font-size: 1 rem;
  To make the text larger, just change
  font-size: 1 rem; to font-size: 1.125rem;, the new code will be
  body {
font-size: 14px;
font-size: 1.125rem;
font-family: Helvetica, Arial, sans-serif;
text-rendering: optimizeLegibility;
color: #444;
}

How to change text of the widget larger?

To change the text,

• Go to widget-area .widget .textwidget
  .widget-area .widget .textwidget {
font-size: 13px;
font-size: 0.928571429rem;
line-height: 1.846153846;
}
• Make change at the font-size.
• Example:
  By default, font-size: 13px; font-size: 0.928571429rem;
  To change text larger, just replace to the size you want.  font-size: 15px; font-size: 0.975rem; and then the style code will look like the following
  .widget-area .widget .textwidget {
font-size: 15px;
font-size: 0.975rem;
line-height: 1.846153846;
}

To change heading h3 at the widget,

• Go to .widget-area .widget h3 where as
  .widget-area .widget h3 {
margin-bottom: 24px;
margin-bottom: 1.714285714rem;
}
• To change the text size of widget title (h3) to the size of 15 pixel, add font-size: 15px;
• To add the background of the widget title (h3) to dark red, add background: #800000;

• The final code will look like this
  .widget-area .widget h3 {
font-size: 15px;
margin-bottom: 24px;
margin-bottom: 1.714285714rem;
}