How to change 'Tagline' font and size

look in the theme’s style.css for this bit:

/* =Header
-------------------------------------------------------------- */

#header {
	padding: 30px 0 0 0;
}
#site-title {
	float: left;
	font-size: 30px;
	line-height: 36px;
	margin: 0 0 18px 0;
	width: 700px;
}
#site-title a {
	color: #000;
	font-weight: bold;
	text-decoration: none;
}
#site-description {
	clear: right;
	float: right;
	font-style: italic;
	margin: 14px 0 18px 0;
	width: 220px;
}

/* This is the custom header image */
#branding img {
	border-top: 4px solid #000;
	border-bottom: 1px solid #000;
	clear: both;
	display: block;
}

set the font size to what you want under this section:
#site-description

try this:
font-size: 20px;