Context/Intro

I'm developing a mobile game that is played in portrait orientation. The player is at the top of the screen (unable to move vertically) and flying objects that either need to be picked up or dodged, move towards the player from the bottom.

Question

Determining the size of the assets/sprites or how much the camera is zoomed in has been a struggle for me and I was wondering if there is maybe a good guideline to follow to avoid common mobile device mistakes like eye strain, touch screen precision, and analyzing whether the approaching object is to be collected or avoided? e.g. if a moving object is too small, the player won't have enough time come to a decision on what action to take.