An ounce of lofty Felt Alive Wool, when rolled up into a ball, will fill a 2 cup measuring container. Depending on how dense or firm you felt, this ounce of wool can stay nearly this size or can be felted down into the size of a walnut. (this kind of size reduction takes many hours, can cause needle breakage as well as hand fatigue as the felt gets really dense.)
Not rolled into a ball and stuffed into a measuring cup - an ounce of Felt Alive Wool Batting looks about like this:
For my projects and my style of felting, a twelve inch doll typically weighs about 3-5 oz. Since they are solid needle felted wool (no wire armatures or sewn fabric clothing) that's how much wool it takes to make my doll. The bulk of my dolls are the core parts that I felt with my un-dyed core wool batting - I plan on using two ounces of core wool for each project and then another two ounces of a skin color. The remaining weight (if any) comes from the wool I use to needle felt the
clothing to the doll and the wool I use for hair.
For a 3 " Pixie, it takes less than an ounce!
I hope this gives you some idea of where to start when shopping for your new needle felting obsession!