The rule of thumb is that screen cannot be larger than 4" to be fully accessible using thumb only. Often you have only one hand available, which makes most of allegedly "awesome" 4.5"+ smartphones inconvenient at best and useless at worst. I don't get this whole 5"+ mindset. If you want something bigger (what for? watching movies?), isn't it better to have 7" tablet while leaving your phone handy?

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I'm user of Galaxy Ace 2 / i8160 with 3.8" screen and 480 x 800 resolution. I'll change it only for something with 1280x768, but with screen no bigger than 4". Good pixel density (i.e. ~250+ ppi) is really important for comfortable reading. And better battery is a must.