While the iPhone is currently available in just two screen sizes, iPhone 6 will reportedly be available in two new sizes, 4.7-inch and 5.5-inch. That gives you a lot of options — 3.5-inch, 4-inch, 4.7-inch and 5.5-inches. So how do you know what screen size is the best for you?

A big screen is great for almost all activities you do on your phone. You can have larger text, the display can accurately show higher resolutions images and HD video, and the tap targets become even bigger, so that you easily hit buttons with a finger.

But, you of course can’t use a phone with a very big screen. One of the most important factors in deciding the feasibility of a big screen is seeing if you’re able to use the phone with one hand. That is, is your thumb able to reach the far end of the screen without you having to stretch your fingers? Apple, in fact, touts that the 4-inch screen of the iPhone can easily be navigated with just one hand, and Samsung’s giant Galaxy Note 3 comes with a special one-handed mode.

The Wall Street Journal takes up this issue, and explains:

If you’re in a store, test phones by loading a drawing app such as the one at Awwapp.com, which works in a Web browser. Hold the phone comfortably with one hand and draw an arc with your thumb, like a windshield wiper. That’s how much of the screen your thumb can comfortably reach.

The other thing that matters is how strong a grip you’re able to get on your phone. You don’t want it to be so wide or slippery that it falls off from your hand if there’s a slight push or jerk.

WSJ made an interactive demo to measure the size of your thumb, and then gives you an idea of how your thumb-reach and grip on various phones would be, based on their sizes. You can access the interactive graphic by going to this link.

So with the possibility to choose from many options very soon in the future, what size iPhone would you choose? Let us know in the comments below.