A number of different contrast options are available which may help some users with visual impairments. Choosing a non-standard option will reload the page with your preferred contrast setting for increased visibility.
For visitors who wish to view the site in larger or smaller text size, modern websites allow you to zoom in and set larger text sizes.
On Firefox clicking the "View menu" provides "Zoom" options to allow you to zoom in either on the whole screen or just the text.
On Internet Explorer the "View menu" provides "Text size" options to allow you to change the text size.