I'm a developer at a small web agency that works primarily with charities. We tend to put a simple textsize widget on every site (eg. top right on www.embraceme.org). I find myself wondering if that's necessary: if someone has difficulty seeing, then there's a number of tools at the OS/browser level to help them out. If each website reinvents the wheel then that's a new tool the user has to keep relearning (not to mention extra and unnecessary complication on the site). We don't have an accessibility/usability specialist, so would love to hear people's views on this.
Providing such font size options within individual websites isn't as important nowadays than it used to be (for instance when IE6 was a more common browser as it didn't really have a suitable font resizing option) but that doesn't mean it no longer has its place; it is particularly useful if the target audience for the website is more focused to users with disabilities - such as for the London 2012 Paralympics.
However your main priority with font sizes is to ensure you put your site together using correct modern HTML standards rather than focusing on minor 'widgets' here and there.
The main accessibility requirement around text resizing is that:
WCAG 1.4.4 Resize text: Except for captions and images of text, text can be resized without assistive technology up to 200 percent without loss of content or functionality. (Level AA)
This can be achieved in a variety of ways, all using good HTML standards. For example, use standard readable fonts, don't set your default font size too small, keep your fonts in
ems instead of
px so that they resize correctly when the user adjusts the font size via the standard browser controls, and keep your layout adaptive / fluid so the content doesn't expand out of the screen / containers when the font size is increased.
Check your target audience for your project. If the users are more likely to benefit from the in-page font resizing due to minor visual disabilities or those that suffer from dyslexia, or (and I hope this isn't the case anymore) if they're locked into using IE6 then yes, it's probably going to be useful to provide that option, but for current web design it's more important to get your code in shape and use good HTML and you can let the users take control of how they want to experience the web.
I recently attended a Nielsen/Norman Group conference and this question was asked. The answer they gave was that in the many usability tests that they have done, very very few, if any, users ever actually used the feature.
That said, if you did want it to be used, placing it in the upper-right location makes it much less likely that a user will find/use it.
You might want to try testing some different placements or configure GA Event Tracking to see if people are using the feature at all.
If you don't set the font size of the main text, it will use the font size specified in the browser settings. It looks like your site does not set this so it will use browser settings font size, making a text widget somewhat redundant.
(So in the future ensure it remains this way, the font size for the main text is not set in the CSS to keep it working the way it is.)
If you built the site in that way that the font-size (*) can be increased/decreased by the user's browser, there is no need for a widget.
*: Note that font-size zoom is not the same as the page zoom (the latter is the default in many browser today, but it can be switched to font-size zoom by the user).
If your site's font-size cannot be changed by the user's browser (because of a special way you built your site, which should be a very rare case), you should provide a widget.
I'd say you include it, not on the right top though. Why? Because older people using the web might not understand very well how to change settings on their browsers and something they can see available right there would be more helpful for them. It doesn't interfere with magnifier software such as Zoom Text, however I don't know how would it interact with screen readers such as Jaws.