You can buy different thicknesses of metallic threads, sometimes it means buying different brands. I use the finer threads for details such as the scales on the dragons and the thicker ones to give a more solid impression such as the sword.
For a fine thread I would use a Kreinik Cord, Madeira FS40 or one strand of a DMC Light Effects thread.
For a thicker thread I would use a DMC Diamant or a Kreinik #4 Braid or you could use two strands of a DMC Light Effects thread.
Lakeside Needlecraft (www.lakesideneedlecraft.co.uk) are really good for their selection of metallics.