You can tune this value with only a single camera image. Just take one (using Frame and Focus) that mimics AF filter, binning and exposure time and then click the "HFR" (star) button.
Detected stars will be circled in green. As you adjust the minimum star size, the detected stars will adjust with it in order to show you what effect you are having on the detection routine.
Did you get to read through this guide:
http://www.mainsequencesoftware.com/Content/SGPHelp/UnderstandingAutoFocus.html
Specifically, the section titled "So... How do you set it up for your rig?"