Hello!
While stretching sprue takes some practice, it's a very good thing to learn - many things can be built out of different thickness stretched sprue lengths. I usually take a zippo lighter or a candle for the heat source, and a nice comfortable length of straight round sprue. While heating I rotate the piece and I also move it along its axis. Initially I'm holding it only with one hand, so that whan it starts to droop I know it is getting soft. Then that's the time to grab the piece with the other hand, heat it a little more, and then take away from the heat source and stretch with a smooth movement. Now the diameter of the resulting stretch depends on how hot the heated piece was, and how fast the initial stretch was. If your first movement is fast, then the result will be a thin fibre, if you go slow you can get a length of a rod like 2mm or 1mm in diameter. Some people go different way - they put one end of sprue on fire, then put it out and then quickly stick the just extinguished end to a metal surface and pull, to get a thin fibre.
Now if the piece you are working on doesn't get long or thin enough, or if it rips, that means you didn't heat it hot enough. Try holding the sprue over the flame longer, but with moving it, so that it does'n catch fire.
Good luck with your builds and have a nice day
Paweł