A radon fan connected to a vent pipe in a crawl space is a great way to remove radon, odors and moisture by moving air only from under a sealed membrane. Many crawlspace systems sacrifice your conditioned air by blowing it outside which will cost you big bucks!
The amount of crawl space ventilation required will likely be quite small if you have done a good job sealing the crawl space vapor barrier. I rarely need to use large vents (4") for crawl spaces unless they are larger than 1000sq/ft or are leaking heavily through old rock type foundations. If you do use a larger vent, a damper or valve can be placed inline to fine tune the suction as needed.
Here is an example of a radon vent setup for an 800 sq/ft basement with a 375sq/ft crawl space.
The 2" line actually reduces down to just 1-1/2" pipe and the membrane was still sucked down tight in the 375sq/ft crawl space. By doing this I direct more suction to the 3" line which requires more pull since there was sand below the main basement floor in this house.
Notice how the pipe slopes downward to the basement floor and not into the 2" line. You don't want condensation to fill up any sections of your vent pipes.
I use a 4" drain tile tee which accepts 4" SCH 40 pipe or a 4" to 3" bushing as seen in the photo above. There is room for the plastic to to be sandwiched between the pipe and the tee. With a little caulk this makes for very secure connection.
When I want to use 2" or 1-1/2" pipe for my crawl space ventilation line, a 3" drain tile tee is used with a 3" bushing for the pipe size.
The drain tile tee is placed under the membrane before sealing it to the walls in close proximity to the vents location.
Here's how it comes together....