This is the standard Microrax screw. M3 thread, 5mm long, in stainless steel.
The length is correct for joining 2mm thick brackets to the extrusion. For any other screw lengths (for other attachments), the thread should extend 2.5mm ±0.4 into the profile groove.
Hex driver is 2mm.
Based on the screw joint calculator, each M3 screw can hold approximately 300 grams.