If the superb Galaxy S5's 5.1-inch screen is too big for you to work with one hand, you can easily enable the one-handed mode on Galaxy S5 and shrink the screen as much as you like.
Do as follows to change and adjust the screen size on Galaxy S5:
1. Open Settings application
2. Use the search box and look for: One-handed operation (you can also find this option here: Settings >> Device >> One-handed operation
3. Make sure the one hand mode is enabled
4. Now, put your finger on the right edge of the screen, swipe to the middle and go back in one motion (sometimes it needs some patience)
5. Press and hold the top left corner of the screen, and resize the screen