Fireline is one of the strongest thinnest cords I have come across. The two sizes I am familiar with are size D and B. Size D is a little thicker than B. D is .008″ and B is .006″. You will need a needle to work with this cord. The size needle that has an eye that will fit both is John James Beading Needle #11.
This combo (needle/cord) is a good choice if you are making a ladder bracelet where your bead holes are very small. The cord itself is barely visible.