The text size in web browsers is caused by the zoom level. The normal zoom level is 100%. A smaller zoom level allows more to fit on the screen, but also makes everything smaller. Conversely, a bigger zoom level means that everything will be larger, thus reducing the amount of text/images that will fit on the screen.
Hold down the CTRL key on your keyboard and then hit the + (plus sign) to make it bigger or the - (minus sign) to make it smaller.
On a Mac, the Command key should work instead of the CTRL key.
You can also hold CTRL (or Command) and use the scroll wheel on your mouse to increase/decrease the zoom level.