From John Palmer's "How to Brew" - "Boil a pint (1/2 quart) of water and stir in 1/2 cup of DME."
From there you can double it. Most information you will find will suggest a 1 liter starter. 1 quart = 0.95 Liters, so basically, you can use 1 quart water to 1 cup DME.