When the digits were all the same font size, Tammet recalled 68 correctly, compared to an average of about eight for a control group, and he remembered all 68 when tested again 3 days later. (sciencemag.org)
Comfortable in the hand (7 ″ are too heavy and generally too wide), comfortable on the eyes (keeping the same font size on a 5 ″ means you're squinting) while keeping a lot of text on the screen (on a 5 ″ you have to crank the font size up a bit, which means less text onscreen and more page turns required). (goodereader.com)