It's a cable release. Don't worry about it. If the thread doesn't go in straight or you only get half a turn in before it stops, just back it out and retry. As long as you don't try to force it, it won't ruin anything.
As for length, it's all personal prefence. For some cameras, like my Sinar F1, I like long cables. About 3'. It's a big camera, and I often like to have plenty of length so I can rest it on the back of the camera without bending it too acutely. For my Speed Graphic, I prefer smaller ones. About 6". I usually trigger it while standing on the side, so I'm only using it to keep camera shake down.
The only thing to worry about is a pinching the cable by bending it at too sharp of an angle. The straighter it is, the easier it will be to actuate. So a longer one might be easier to use because you can do more gradual bends. But if you're using it straight, then that's not an issue. Also, you may or may not want one with a lock. This allows the shutter to remain open without holding the button down. It's not really useful for the Speed's focal plane shutter, but some leaf shutters have to be held open while focusing and composing with a cable release, and a lock comes in handy for that. Also it's good for long exposures is your shutter doesn't have a T setting. Most shutters have a switch or T setting or switch to hold the shutter open for focusing and composing, so it's not usually a necessity.
If you have an old cable release that doesn't function well due to too much internal friction, spray some silicone lubricant down inside and work it a bunch of times while letting the lube drip through. I've saved several old cables that way.