Before asking people to take the time to answer your questions, you might do some research on your own, or at the very least read the responses you've already gotten. Here's my post on your thread from 2 days ago.

....You can either replace with the same size as original, or you can change provided you also change the ferrules to ones with the correct ID. Also note that Index housing ferrules are different than those for brake housing, having a thicker flat bottom (inside) vs. thinner conical bottom for brake. (index ferrules can also be used for brakes).

As to the logic for different OD,s thicker is stiffer and durable, thinner is lighter and makes a smaller lump under tape, use whatever you prefer, doesn't really matter.

I prefer to keep things simple, so I use 5mm brake and index housing and use 5mm brass index ferrules for both.