You starter needs to be 2 liters (half a gallon) of 1.035ish wort. Here's John Palmer on the topic:
Making a Liquid Yeast Starter
Liquid yeast packets should be stored in the refrigerator to keep the yeast dormant and healthy until they are ready to be used. There are two types of liquid yeast package - Those with inner nutrient packets and those without. The packages that contain an inner bubble of yeast nutrient (i.e. a "smack pack") are intended to function as a mini-starter, but are really not adequate. They still need to be pitched to a starter wort after activation. The package must be squeezed and warmed to 80°F at least two days before brewing. The packet will begin to swell as the yeast wake up and start consuming the nutrients. When the packet has fully swelled, it is time to pitch it to a starter to increase the total cell count to ensure a good fermentation. I prefer to prepare all my liquid yeast packages yeast four days before brewday.
1. If you are going to brew on Saturday, take the yeast packet out of the refrigerator on Tuesday . Let it warm up to room temperature. If it is a smack pack, place the packet on the countertop and feel for the inner bubble of yeast nutrient. Burst this inner bubble by pressing on it with the heel of your hand. Shake it well. If you are not using a smack pack, proceed directly to step 3. You will be making two successive starters to take the place of the mini-starter smack pack.
2. Put the packet in a warm place overnight to let it swell. On top of the refrigerator is good. Some brewers, who shall remain nameless, have been known to sleep with their yeast packets to keep them at the right temperature. However, their spouse assured them in no uncertain terms that the presence of the yeast packet did not entitle them to any more of the covers. So, just put the packet somewhere that's about 80°F, like next to the water heater.
Figure 36: After about 24 hours, the packet has swelled like a balloon. Time to make the yeast starter.
3. On Wednesday (or Tuesday for slants) you will make up a starter wort. Boil a pint (1/2 quart) of water and stir in 1/2 cup of DME. This will produce a starter of about 1.040 OG. Boil this for 10 minutes, adding a little bit of hops if you want to. Put the lid on the pan for the last couple minutes, turn off the stove and let it sit while you prepare for the next step. Adding a quarter teaspoon of yeast nutrient (vitamins, biotin, and dead yeast cells) to the starter wort is always advisable to ensure good growth. It is available from your brewshop.
4. Fill the kitchen sink with a couple inches of cold water. Take the covered pot and set it in the water, moving it around to speed the cooling. When the pot feels cool, about 80°F or less, pour the wort into a sanitized glass mason jar or something similar. Pour all of the wort in, even the sediment. This sediment consists of proteins and lipids which are actually beneficial for yeast growth at this stage.
Ideally, the starter's temperature should be the same as what you plan the fermentation temperature to be. This allows the yeast to get acclimated to working at that temperature. If the yeast is started warmer and then pitched to a cooler fermentation environment, it may be shocked or stunned by the change in temperature and may take a couple days to regain normal activity.
5. Sanitize the outside of the yeast packet before opening it by swabbing it with isopropyl alcohol. Using sanitized scissors, cut open a corner of the packet and pour the yeast into the jar. Two quart juice or cider bottles work well, and the opening is often the right size to accept an airlock and rubber stopper. Cover the top of the jar or bottle with plastic wrap and the lid.
Shake the starter vigorously to aerate it. Remove and discard the plastic wrap, insert an airlock and put it somewhere out of direct sunlight. (So it doesn't get too hot in the sun.) If you don't have an airlock that will fit, don't worry. Instead, put a clean piece of plastic wrap over the jar or bottle and secure it loosely with a rubber band. This way the escaping carbon dioxide will be able to vent without exposing the starter to the air.
6. On Thursday (or Wednesday for slants) some foaming or an increase in the white yeast layer on the bottom should be evident. These small wort starters can ferment quickly so don't be surprised if you missed the activity. When the starter has cleared and the yeast have settled to the bottom it is ready to pitch to the fermenter, although it will keep for 2-3 days without any problems. However, I recommend that you add another pint or quart of wort to the Starter to build up the yeast population even more.
The starter process may be repeated several times to provide more yeast to ensure an even stronger fermentation. In fact, a general rule is that the stronger the beer (more fermentable/higher gravity), the more yeast you should pitch. For strong beers and barleywines, at least 1 cup of yeast slurry or 1 gallon of yeast starter should be pitched to ensure that there will be enough active yeast to finish the fermentation before they are overwhelmed by the rising alcohol level. For more moderate strength beers (1.050 gravity) a 1-1.5 quart starter is sufficient. One consideration when pitching a large starter is to pour off some of the starter liquid and only pitch the yeast slurry. One recommendation when pitching a large starter is to chill the starter overnight in the refrigerator to flocculate all of the yeast. Then the unpleasant tasting starter beer can be poured off, so only the yeast slurry will be pitched.
