Bullet crimp connectors likely about 4 or 5mm.
Personally one is better off soldering direct or using a better quality connector like an XT, Powerepole (Anderson) or Deans.
Only if you can't solder, use the brass/copper bullets. The weak point is the insulation, heat can melt it overtime and this is where any frying will take play as they short. Use heat shrink over the wire joint and then the insuation sleeve.
Copper or gold soldered connectors with decent insulation separation are best.