Like most in the Android world, I've been steadily increasing my comfort zone on how big a screen I want. Back in the day, I was obsessed with getting my phone as small as possible, like Zoolander. Then I got my first smartphone in the Windows Mobile 6 days, and ever since then every device I get has a bigger screen than the last, and I end up being happy about it.
There are a couple of sizes that are natural points for a mobile device's size. The two big ones that jump to mind are:
The biggest size that you can easily reach the whole screen with one hand, and
The biggest size that you can comfortably fit in your pocket.