In many of the recent browsers, if you hold down control and–using the scroll-wheel on your mouse–scroll toward yourself, the text size will zoom up. Likewise, if you scroll away from yourself it decreases the text size.
Some browsers support key commands as well. By holding down control and pressing the '+' key, you can increase text size and by holding control and pressing the '-' key, you can decrease text size. Also, many browsers have text size options in the 'View' menu.