I was reading the book Yeast just the other day, and they have a chart with different inoculation rates to show that starter growth is dependent on starter volume and yeast cells pitched (not just one or the other). A quart starter, which is roughly 1 liter, with 100 billion cells pitched will grow by roughly 50%, resulting in about 150 billion yeast cells.
The MrMalty calc is definitely a handy tool to use, too.