Yingying - I sent you a private message about books.
While I have worked with a lot of different fibers, I have my own favorites and I tend to work with Merino more than any other fiber. Merino is often not the favorite fiber of needle felters because it does require a bit more work (oddly enough, Merino fibers wet felt fast than most other fibers).
Which fiber to use is often just a personal choice. I prefer Merino because it produces a lovely smooth finish. I use other fibers for specific purposes such as Shetland for beards on small gnome figures that I make. The Shetland has a fine crimp to it and is just perfect for beards! It also makes for a very fine fuzzy surface similar to the texture you can see in my Happy Cat posted over in the Finished Projects forum.
Making objects of equal size is a matter of measuring the fiber. Fiber that has been carded and aligned to look rather like a long rope is fairly easy to measure. For example: you may start with a piece of rope fiber that is 8" long. That can be divided in half (lengthwise) so that you have 2 portions of fiber that are each 8" long. Those could be divided again and you have 4 pieces of similar size 8" long. There may be minor differences in the quantities which might become evident when the fiber is felted. The differences are apt to be small and you can correct any differences with small additions of fiber.
The batt fiber that I use is easy to pull apart and I can measure it easily as well. Generally with batting I'm working with sections of fiber that are 1" wide by 4" long or whatever I think I need for a project, but the way the batting is constructed allows me to easily pull off another piece of batting that matches the dimensions of my first piece of fiber.
When making objects that need to be of equal size - measure the fiber for all of the matching pieces at the same time - before making the first piece.
How much fiber is needed for any piece depends a lot on firmness. If you do not need a piece to be very firmly felted, it will require a lot less fiber than a piece that needs to be very firmly felted.