The main factors that influence the radius of a level and coordinated turn are the bank angle and the airspeed.
In general, to achieve a minimum radius (level) turn one needs to increase the bank angle and to reduce the speed as much as possible. However, the stall speed, load factors, and available power might dictate some practical trade-offs in order to achieve the minimum possible turn radius.
You can find a more detailed discussion about this topic in the following article: How To Safely Reduce The Radius Of Your Turn—In Case You Need To.
It is relevant to note that the knowledge and the practice of maximum rate turns (minimum radius) can be an important safety factor especially if you ever need to take an evasive action which requires a tight turn while maintaining altitude.
The mobile application allows you to quickly estimate the turn radius given the selected bank angle and safety margin above stall speed to execute such turns. Have fun!