Diameter of 8/0 threads is approximately 70 to 90 denier. Strength varies by materials with GSP threads being far stronger than traditional cotton threads which is again far stronger than fly tying silks.
An 8/0 fly tying thread is agood thread for small to average trout flies and for larger flies where bulk is an issue. It makes small neat heads on flies. If bulk is an issue then look at Semperfli Nano Silk in 30D 18/0 and 50D 12/0, these threads lay flat and are 3 times stronger than any comparable thread, it has the same breaking srain as Kevlar threads!