The useful range of resistance is probably somewhere around 4 to 10Ohms, with 7 being a good compromise between output power and safety at 12V (a 4Ohm heater heats the extruder up to 240C in 20 seconds, but will quickly reach more than 500C, should the microcontroller hang up without a proper watchdog, while 7 Ohms will ruin the extruder as well, but should not set it on fire).
Whatever wire you have will do, just use an ohmmeter to measure the proper lenght and cut it 2cm longer to allow for a pair of crimp-on connectors or knots. AFAIR, nophead is using 0.1mm-diameter wire, while I'm using 0.21mm (AWG32) salvaged from an old hairdryer that was about to hit the bin. There's usually 5-10m of this stuff in a typical hairdryer. It's coiled and requires some minor effort to straighten it properly, but given the cost (none or marginal), it's probably the best source.
When taking the heater coil out, take care not to pull on it, do it gently. It will be in tightly coiled sections connected by short, semi-straight runs of wire. Cut it at those points and either put each cut section on a smooth steel rod with a matching diameter and pull on one end away from the rod while pressing on the other end, or imagine that it's wound on such a rod and you need to undwind it, turn by turn. You will get a run of pretty straight wire with some tendency to form large coils - nichrome is somewhat springy - but without the "wrinkles" you'd get by just pulling the ends apart. That's important if you plan to wind the wire in tight turns later.
Edited 1 time(s). Last edit at 04/08/2009 02:18PM by Enleth.
