What elcreative said.
However, if it's of any help, I have a Sony T1. A document that was originally set in 8pt Times I estimate is roughly around 26pt at the largest font size in 'Original'. If you open Word and set piece of text to that size, you'll get a rough idea. 8pt, BTW, is quite small - most books that use Times are set at 12-14pt, so the increase from this original size would be correspondingly larger. For comparison, 'large print' is defined as a minimum of 16pt, and most are 18pt, so the large font of my Sony is considerably larger than a normal large print book.
The Royal National Institute for the Blind did a comparison of various e-readers and reckoned the Sony (T1, I presume) went up to 36pt. You can download the comparison table from their website:
http://www.rnib.org.uk/livingwithsig...nts/Comparison of ebook reader features.doc
It might be worth (assuming your relative is housebound and can't see these for themselves) if you were to see what letters they can read on a size chart (there's one here:
http://www.printingforless.com/Color...ign-Guide.html), and then compare the fonts on various readers against that size chart so you get an idea of what ones might be useful.