Your hair is long and will shrink considerably. Unfortunately, everyone is different, so we cannot tell you how much it will shrink... Only that it will definitely benoticeable. Hari is probably right though. 50% would be a rough guess. But you will still have some longish dreads when it does.
How long will it take to dread? As I said, everyone is different, but from your description it may be very fast indeed. However, it takes a year on average to grow matured dreads, regardless of the method you use.
If you want to TnR to speed up the process a bit, then wait a couple of weeks for sections to naturally form. Then, decide if you like the size of the section and do the TnR. Keep in mind, the largest you want a section to be is about one square inch. When a dread matures, it will shrink in length, but grow in with to roughly the size of the section at the scalp. If you looking for thin, then pull the bigger ones in half.
It is worth mentioning, however, that it is best to have both big and smaller sections. This looks best (more natural) and is healthiest for your scalp.
Whatever method you choose, relish the journey.