Where could I download a large font calculator?

I believe this is something you can change in the Control Panel on Windows, but I’m not sure: take a look under accessibility.

If you are using a Mac, you can hold control and scroll the mouse wheel (or two fingers on the trackpad) to zoom a particular section of the screen to a larger and more legible size. There are also advanced accessibility options tucked away in System Preferences, if you type ‘accessibility’ into Spotlight (click the magnifying glass in the top right of the screen) OS X will help you change options as you require.