What is the maximum diameter of a pile on which we can perform a dynamic test with two sets of strain transducers?

There is no maximum diameter rule. Two sets are generally sufficient for any diameter pile. However, putting the sensors 2 diameters below top (or even 1.5 diameters) becomes harder as the diameter increases. Also, using two sets allows evaluation of bending in only one plane. If you need evaluation of bending in two planes you need four sets (and bending becomes more of an issue when the pile is not driven vertically). The closer the sensors are to the pile top (certainly if less than 1.5 diameters) the more it becomes important to use four sets. Also four sets gives some backup in case of some difficulty, so is some extra insurance for critical testing of important test piles.