I don’t think there is an absolute answer because of all the variables... Ultimately, the number of fans and the size you end up using must get the radon level to an acceptable range during all seasons. Here is a popular radon fan to consider starting with:

Fantech Rn1 Radon Fan

Continual testing is required and it may be necessary to increase the fan CFM based on the actual test results. Using a digital radon detector is a great way to accomplish continual testing and here is a popular unit:

Digital Radon Detector

yes those pesky variables...

I am looking into hiring a radon mitigation contractor to look into it. Thanks

Just know that this is a relatively easy DIY project, especially if you already have the pipe through the slab and into a properly sized cavity. The important thing is to do continual testing to ensure that you stay under the recommended limit during all the seasons. And put a manometer on the pipe so you always know the fan is operating. I think you will be okay with just using the aforementioned Fantech Rn1 radon fan (20 watts, 169 CFM) given the relatively small size of your slab unless you have an unusually high radon level.

I bought the digital radon detector. We will connect the pipe up and out a wall at the top floor (we haven't done that yet...). Seal the perimeter of the slab again. Maybe a passive system will work. If not we will connect the fan.
Thanks for your input!

We often use stack effect to good advantage in our passive solar cooling/heating building designs. However, I don’t think stack effect will be effective for continuous radon mitigation because the pipe will not have sufficient area and the temperature differential will be variable or too small. Here is a stack effect flow rate calculator based on ASHRAE methods:

Stack Effect Calculator

Anyhow, I am pretty sure the aforementioned fan would be adequate if you need radon mitigation and continual testing will provide you an answer you can be very confident about.

The digital meter is the right choice. Works great and can be moved around. I would check different areas for 1 week period each and record the data to compare.

I also did some upstairs readings.

I would love to see before and after readings and changes when you move it around. I wish I had recorded mine as I moved it so I could share the data. Mine was never close to the limit so I didn't bother.

Thanks for all the advice. I'll take the radon measurements before and after and post them.

One question - under our slab the build-up is: compacted sand, foam - the foam was sliced to accommodate the two runs of perforated pipe - then poly membrane sealed to the foundation walls. Because the substrate is sand I am wondering if we might need a stronger fan.

We may get our answer by seeing how well our passive system works...

I don't remember code but we had to put gravel and one pipe with a tee on the end down into the gravel... no runs of PVC or cuts in the xps... just a layer of plastic under the insulation...

It was never checked or the levels checked by any code official. We checked ourselves and were half the limit.

Good luck.

If you have an existing house and then subsequently discover that you need radon mitigation, they just cut a hole somewhere in the slab, remove about 5 gallons of under slab material to create a cavity, place a vertical pipe in the cavity and seal the slab around the pipe. So in this case, there is no under slab horizontal pipe at all... While I would fully expect that having a long perforated horizontal pipe surrounded by gravel under the slab would be more effective, apparently just having a cavity that is kept at low pressure by the fan will draw the radon to that path and successfully accomplishes radon mitigation to some extent.

I should say that I am not a radon mitigation expert or even in that line of business. I am just conveying what I have seen done over many years by the radon mitigation experts and DIY homeowners. Again, testing will tell you if you even have a radon issue and will then tell you the effectiveness any mitigation that you subsequently accomplish.

I would personally start with nothing, then the aforementioned 20 watt fan and then work up from there if necessary based on the testing. Yes, please post your results as this may be insightful for others.