I think it depends on what you are rebuilding. I would suggest watching superxdrifter's video on building micro
coils for an rm2. It is somewhere in the massive microcoil thread. This translates well to other builds. Scubatadan made a good one also. 28-30 ga kanthal or nichrome is a good starting point, wrapped around a 1/16" drill bit. Get a multimeter to you can test resistance between the posts before firing. Don't forget to test the internal resistance of your multimeter and subtract that from your coil reading. If you are concerned with safety, don't go to extremes, like sub ohm
coils or large gauge wire. 1.8to 2 ohms performs well on most equipment, and is safe enough for nearly any battery. Seven or eight wraps of 30 is well within that safe zone. Position the coil so it isn't touching anything.
There is a lot of great info here in this forum, and lots of helpful, knowledgeable people. For more specifics, let us know what you are building on. Good luck!