I'm going to take this a step further, though. Because the ideal button size for a touchscreen is actually .75" x 1.25". Why? For the same reason your widescreen TV is more pleasing to the eye than the set you grew up with. (It has to do with proportion. I'll explain the science behind this in a minute.)

Okay. So size matters. But should you worry about setting the buttons too close for comfort? Interestingly, the gap, or white space, between buttons has little impact on user performance and accuracy. The biggest factor really is size. So if screen real estate is at a premium, you can place the buttons right up against each other. Still, if you have room, try for a gap of up to .125". While this won't increase user accuracy or speed, it will increase the user's comfort level when working with your interface.

Want smaller buttons for your design? No problem. It's the bounding box of the button's active touchable area that's the biggest consideration. If you want smaller buttons, increase the gap between them and consider making the active touchable target of each one extend beyond its visible area.

If you're working with a small touchscreen display, you may be forced to go with smaller buttons. A button size of .375" is acceptable, but never go below .25". Imagine throwing darts. It's difficult, no matter how close you are to the dartboard, to hit the bulls-eye every time. And when the target is a touchscreen button, you can't expect a finger to hit the center every single time. A person with average dexterity touching the same point multiple times will only be accurate within .125".

What about going even bigger? Well, going bigger actually doesn't increase performance and accuracy, and can actually slow the user down because the finger has to move a greater distance with each touch.

Now for the science behind that ideal button.

Phi is known as the Golden Ratio. It's a mathematical constant like Pi. Except Pi (which is roughly equal to 3.14) has to do with circles and Phi (which is roughly equal to 1.618) has to do with line segments. Since the Renaissance, Phi has been considered by many to be a measure of the perfect proportion. In fact, so many examples of the Golden Ratio can be found in the Mona Lisa that you could literally write a book about it. 

And if you apply the Golden Ratio to button size, it turns out that the ideal button is .75" x 1.25" instead of .75" x .75".

Here's how you do the calculation: .75" x 1.618 is roughly 1.25".

Morgan Strauss is Director of Operations for Guifx, developer of touchscreen interfaces. He has 16 years of experience designing digital and analog interfaces for the aerospace, medical, automation, and entertainment industries. He's taught many classes and seminars, including 'Interface Design for Touchscreens' at the Consumer Electronics Show (CES). For more design tips visit the Guifx Blog