What's the smallest 3-blade propeller for this motor? I've got a 15470 brushless 35kw motor from China
How do I find the smallest 3-blade propeller I can use for this motor?

Here are it's specs,
Max Power: 35KW
Max Amp: 400A
Max RPM: 8500
Max Volt: 120v
Torque: 75Nm
Resistance: .94

I've looked into eCalc but it's for determining power / thrust, etc and not finding prop size.
Any help would be appreciated!
You need the largest prop that keeps the tip speed below about .9 Mach, roughly 1000 ft/sec, so you want the largest propeller than can operate at that RPM at .9 mach.  Here's a tip speed calculator for larger propellers that should suffice.