Pipe selection is a critical component of a radon system. Ultimately this falls on the discretion of the mitigator for each application. However, there are several things to keep in mind.
The bigger the pipe, the harder it is to work with and to "hide". On the other hand, the larger the pipe diameter, the more air it moves. This "balancing act" must be weighed each time by the mitigator as they make their choices.
Remember that the best pipe size for the mitigator may NOT be the best size for the customer! I have seem competitors that routinely "always" use 3-inch pipe! Larger piping may cost more, but it may at times be "better" for overall system performance! The type of "medium" encountered under a slab foundation will also impact the size pipe that should be used! Have your mitigator explain in his system proposal WHY he has chosen a given size pipe for your system!