maybe I can help you. I have a step-dad with trouble seeing and so he needs a basic usb keyboard with LARGE print and a Magic Keyboard cover also with LARGE print. The design of the darn things has changed a bit, so rather than needing batteries, they have some kind of built in battery and a Lightning socket on 'em. Anyways, I ordered one from Amazon ( which was the basic USB keyboard) and a place in the UK (for the Apple Magic keyboard) . I think amazon also has Magic keyboard covers as well. If you go to Apple's webpage for that thing, I think it'll tell you the length and width of the keyboard, if not you can always look it up somewhere, and then match that to the length and width of the keyboard cover.One "gotcha" I found was if the keyboard had a numeric pad on it or not--some do and some don't, and the other one was the shape of the enter/return key on the keyboard. Apparently European keyboards have this funny-shaped one vs the rectangular one we use in North america, not like it makes a huge bit of difference, but if you're ordering a LARGE PRINT keyboard cover for your Apple Magic Keyboard, you want the darn thing to fit 100% right off the bat, and the shape of the return or enter key in the keyboard cover, when you put it on your Magic Keyboard, and you find out that no, they are not standard all over the world, there's the other "gotcha" and the darn thing won't fit. Here-lemme see if I can get that info for you: it's 10.98 inches wide (27.9 cm) and 4.52 inches deep (11.49 cm) , and that's for the latest one, not the wireless keyboard from 2003,2007,2009,2011. Here's how to tell 'em apart
How to identify your Apple wireless mouse, keyboard, or trackpad - Apple Support
here's one place to try
http://kbcovers.com/large-type-keyboard-cover/
and here's another
http://logickeyboard.com/shop/mac-skins-986c1.html
I went with amazon and a place in the UK... just my experience... but I have found that not all Apple keyboards are exactly the same
hope this helps you out some
John B