I tried the game and the buttons are too small and too close to each other for me, same for title screen buttons. I really want to like the game but it is a pain to control!
Sved, via Distractionware
Established research on touch hit areas is a good starting point. 2.4cm has been found to be the ideal size for touch screens, but impractical for smaller phone screens, so 0.96cm is the recommendation for phones.
Bear in mind that people with either reduced accuracy or low vision will find even this difficult so use them as a minimum rather than a fixed size to aim for. Buttons also shouldn’t be close to each other. If they are, they need to be larger again to compensate.
This also applies to cursor driven interfaces (mouse/analogue stick/motion detecting), where gamers with impaired precision will also have difficulty accurately targetting small elements.
Best practice example: Flappy Bird
Best practice example: Rush Rally