I have a large resistor that goes in my J-Head extruder. It's grey, and it came with the extruder. I'm uncertain as to what grade of wire I need to solder to it. It being one of the elements of the system that heats the hottest, I would think that it would be important to find out what sort of wire is the correct kind to use on it, since heating elements require a lot of electricity.
Also does the solder type I use matter?
The physical size of the resistor is irrelevant. What matters is the total power (and in particular, current) the head will draw. Once you know that, there are plenty of tables on the web that'll list max amperage vs. wire gauge. When in doubt, use a larger wire :-) since we're only talking about a couple of feet length; cost is minimal.
As to solder: so long as you're not selling the printer, you don't need to meet any RoHs requirements. Use whatever you want.