There is no way to modify the default font size from the user interface, so you will need to customise Confluence's CSS manually. This can be modified in your Confluence install directory under \confluence\WEB-INF\classes\styles\site-css.vm

The first entry is:

body, p, td, table, tr, .bodytext, .stepfield {
	font-family: Verdana, arial, sans-serif;
	font-size: 11px;
	line-height: 16px;
	color: #000000;
	font-weight: normal;
}

By changing the value of the font-size field, you will be able to increase or decrease the base font size shown in the browser.