You have three choices for securing your iPhone:
1. No passcode
2. A simple, four-digit passcode
3. A longer passcode for added security
The last option is obviously your best option if security is of the utmost concern, but entering a long passcode on the alphanumeric keyboard can be laborious. Unless... you choose only numbers.
If you create a long passcode that consists of only numbers, your iPhone will give you the numeric keyboard, which has fewer and larger keys than the alphanumeric keyboard.
To set a longer passcode, go to Settings > General > Passcode Lock. Next, turn Simple Passcode to Off and tap Turn Passcode On at the top of the display (if you aren't already using a passcode). Enter a passcode of any length and the next time you go to unlock your phone, you'll be able to enter your passcode with the numeric keyboard.