Designing for Large Screen Smartphones (Luke Wroblewski)

Designing for Thumbs: 
In his analysis of 1,333 observations of smartphones in use, Steven Hoober found about 75% of people rely on their thumb and 49% rely on a one-handed grip to get things done on their phones. On large screens (over four inches) those kinds of behaviors can stretch people’s thumbs well past their comfort zone as they try to reach controls positioned at the top of their device. (lukew.com)