But How Much do I need? Resin and Silicone

Please take a good look at Mothi's Just This And That weblog.
I believe she thoroughly covers all of your questions about the kinds of materials, and how much you need.
Start small, and work with small quantities. That way, after you gain some experience, you will be ready to work larger.

A small fail does not cost as much as a large fail.
When you are learning, expect to fail.
You will make mistakes at first.

Many of the molding and casting materials are measured by volume.
If you will keep a studio notebook of everything you use and everything you do,
then after a while, you should get a pretty good idea of how much you will need.

Otherwise, there are so many variables involved, that it is almost impossible
to say that a 70cm BJD is going to require so much material, and a 1/4 BJD is
going to require this much material. At the very best, you might be able to suggest
a range of needed materials for a given doll, such as 1 to 2 quarts of rubber for molding
a certain sized doll part, or whatever. It all depends on how you make your molds,
and how thick the walls of the resin casting are, and so forth. There are ways
to make molds that require less rubber than others. A lot depends on the cores you make.

When we estimated molding materials for making sculpture molds, we always made
10% more than we thought we would need, because it is always better to have a wee
bit left over, than to not have enough to complete the mold. When you get really good,
you will find yourself pouring a mold, and running out of rubber just as it reaches the
top of the mold box. That is a really good feeling. We always kept a mold-in-progress
of something that wasn't critical, in a corner of the studio, covered, so we could pour any
excess rubber over it, if there was any excess. That way, waste is at a minimum. Rubber
is expensive.

Look through twigling's posts about molding and casting her BJD, here at The Joint
to get a better idea of what is involved, the amounts and kinds of materials needed,
and so forth.

Have Fun !!!
(^_^)