The quick recommendation: For the biggest text, go for the biggest screen (see, it’s not rocket science after all).
The longer answer:
One of the main reasons I like Android as a large print user are the larger screens available. Android phones have screen sizes ranging from 3” up to over 6” (measured diagonally like a TV). In contrast, the newest iPhone 5 has a 4” display, before that it came with a 3.5” display. The advantage of a bigger screen is that without doing anything else, the text on a Galaxy Note II (5.5” screen) for instance is 63% larger than equivalent text on an iPhone 4s.
Some people don’t like to have such a large phone as it can be harder to fit in a handbag or pocket, and harder to use one handed, particularly if you have smaller hands. I’d still argue the merits of at least a 4.5” screen.
While it’s best to compare phones in person, and 3.5” and 5.5” can be just numbers on a blog, there are web sites which will compare phone images next to each other scaled to your monitor size so you can see just what the difference is. Several are:
Aspect ratio is how wide something is compared to how high it is. The standard 6” x 4” photo has an aspect ratio of 3:2. A4 paper is just slightly squarer than that. Televisions and computer monitors initially used 4:3 but now mostly use a “wide” aspect ratio of 16:9 – that is, for every 16 inches (or centimetres) the screen is wide, it is 9 high.
Mobile phones and tablets vary from the almost square 5:4 to 16:9. There is no set answer as to which is best. A phone with a narrower aspect ratio, such as 16:9 held in portrait (upright) mode is easier to use one handed and in landscape mode you can read a longer line of text, which is great when using large print, however a squarer 4:3 ratio gives better ‘perspective’ to show more of the text above and below the line you are reading, and in portrait mode it’s easier to read large print. All my smartphones have been 16:9 so I haven’t had extensive experience with both for a personal comparison. On a PC I was slow to move from 4:3 because I see much better with my left eye so a squarer image fills my field of view more naturally whereas a wider image may work better for someone with proper binocular vision.
A couple of interesting articles if you would like further reading:
The main consideration for a large print phone is screen size – the bigger the better, though be sure and check that you are comfortable with the size of it. Aspect ratio can make a difference but often isn’t the biggest deciding factor.