Madog recommends a 100ohm resistor. I used a 120ohm, just because that was what we had lots of spare. The resistor is only in the circuit to prevent from burning out the led. I found that out with my V1.0 usb. I didn't use one and the led burnt out in 24 hours. Btw, the led has a + and - side. Normally the longer leg is the +, and will not work if connected the wrong way.
As for wire gauge, i used 24 awg wire, again because that was what we had most of spare. I used a single strand (blue wire) out of some old phone wire to connect to the center post on the battery/atomizer connection then connected the 24 awg to that.
I built the internal stuff first, then brought in the usb cable and attached the live to the switch and hooked up the ground wire and shrink tubed the joins. You really can't see it too well but the power feed goes from the switch to the resistor to the + leg of the led. Then i soldered the - leg of the led straight onto the outer casing of the battery connector.
If you want an idea for a box i'd look at the 2x AA battery enlcosure. The battery contacts pull out easy, are really compact and are only $0.99.
Happy modding

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PS - Loads of info in the modders forum. If you haven't already, browse through that forum. You'll be making your own Magnum mod in no time !!