18t has been perfect for my 2650 turnigy trackstar on a stock spur, with a 150A EZrun ESC and 2s lipo. I run 3 fans, but really they aren't needed at all! It's just me being cautious after my last motor died. I'm fairly new so I'm probably thrashing the old thing quite a lot, with plenty of stop starts. My shell is also a nitro one and so the precut holes create plenty of air flow
Temp gun says without fans:
Esc temp 32c
Motor temp 36c
Battery temp 41c
And with fans the motor changes to 28c!