What is the fan model on the S3. Have to remember there are lots of people out here that do not own S3's and do not have those fans.
No idea, though I have seen it mentioned in this thread (if not forum).
Also if you buy a 2000 rpm fan it will probably run at max speed all the time. So if there is a 3000 rpm fan and a 2000 rpm fan each rated at 43dB the 3000 rpm model will be more quiet because it has a little more room to keep temps in check.
You'd be wrong in making that assumption. The Noctua (3000 rpm) may have to run at full pelt as opposed to the Silverstone (2000 rpm) because the latter has higher CFM, i.e moves more air faster, thus it will move more air at lower rpm's than the Noctua at full pelt.
You also really can't go above 120mm in size. If you do the fan will be larger than the whole miner itself and be just wasting air.
I am not completely sold on the wasting air argument, suffice to say the thermalright is a 140mm fan with 120mm mounting and it works just fine for me mounted at the front in a pull configuration. Note that you may need to trim it if you want to deploy in a push configuration if you have a chunky ethernet cable.
Just a note about my observation on this, if you mount one of the suggested fans in a pull configuration and run them at full pelt (because they are less noisy), then you can run even the stock S5 fan in a push configuration on PWM. That way, the S5 temps will be low and the S5 will lower the RPM's on the stock / push fan. I was surprised after installing the thermalright that the S5 was lowering its RPM so I disconnected the blue wire (disabled PWM) on it and now the stock S3's whine (on PWM) in push is barely audible over the swoosh of the thermalright.