Just about every person who has visual disabilities have thier computer set up in a number of ways, such as magnification tools and having the browser override all styling even have it use their own style sheet .
With this in mind ,you do not really need to do much to make your site accessible with a special spread sheet for visually impaired people. IF you follow just a few rules you should be fine.
First is contrast, most people with visual problems need contrast. If your default style is not very contrasting then simply have a line at very top of page that is in contrast with a "switch colour scheme" or "Accessibility". Make sure your colours do not clash for people with colour blindness. Here is a good website to visit about colour blindness. You can also test your pages to see how they look to someone who is colour blind. avoid light text on dark backgrounds. Black on white is perfect.
Vischeck: Home
Another thing you should do is to have font sizes in EM not pixels, that way the person viewing the site can control font size in a number of ways.
Keeping with fonts, One of the features you can employ is a text size increase decrease, simply click a link which will load a new style that changes the base font size.
make sure you keep all your graphics and images etc in your style sheet an all content in the document. Why not have your accessability spread sheet remove all graphics and styling and just have a plain text version?
IF you have kept all your graphics as background images etc in styling and have any heading tags or other text declared as disabled, then all you need to do to is load a new style sheet that does away with graphics and allows your hidden heading tags etc to be displayed. A click of a link is all needed to switch seamlessly between layouts.
Make sure your content flows in a good order, label any table headings you use, make sure any images you want to display as content have appropriate ALT tags. This helps out with people who use text readers etc.
Breaking your layout is inevitable with hue font sizes, most people who use huge fonts accept this is going to happen, just make sure that your text can still be read clearly. WHy not make your accessible styling flexible enough to still look nice and unbroken feel when viewing large sized fonts.
Anyway i hope that gives you some food for thought.