Hm, a resistor at the gate? I have the 4.5Volt at the LED even with the gate not connected to the Arduino.
Source voltage? You mean the 12Volt that go to the LED? The IRLU8726PBF is made for up to 30Volt.
But if you look at eldoLED POWERdrive they write 3 meters to the LEDs.And they write 0.2mm2 per 1 ampere.