Note that this only adjusts the font size of the content in the "sidebar" as defined by your WP theme. In my case, using Sylvia Eggar's "Accessible" - a refork of the Twenty Eleven theme, the sidebar consists of the actual right column sidebar as well as part of the header (the tag line) and the footer content. The body of the page - all of the blog entries are not affected by this widget. So, I am not sure if this is all that valuable.
Let me end by proclaiming my two cents: I believe font re-sizing widgets of all types are misguided. Folks who need enlarged, "zoomed" text either know how to do this using the accessibility features built into their OS or already employ the use of special assistive technology to accomplish this. Often these add-on widgets get in the way and generally create bloated webpages.
I'm temporarily using this app on my WP blog for experimental purposes, not as an endorsement of the app.