Your starter size will vary with the cell count you are shooting for, which in turn will be determined by the original gravity of the wort, batch size, and whether it is an ale or a lager. Use Jamil Zainasheff's pitching rate calculator to determine how much yeast to pitch.
As an easy to remember rule of thumb, use 10 grams of dry malt extract for every 100ml of water. This will give you a roughly 1.040 starter, which is good for growing yeast even if your wort will be higher or lower.
Starters made from healthy yeast packages will be ready to use in 12-18 hours, just pitch the starter or build it up again with more DME+water.
Using a stir plate will pull more oxygen into the starter, which will give you more yeast reproduction and get you to a higher cell count.
Too little yeast can lead to longer fermentation times, under-attenuation, and off-flavors from stressed yeast.
Liquid yeasts (vial or smackpack) can be treated the same, and dry yeast does not require making a starter (just use the correct amount. It's cheaper and easier to buy more dry yeast than to build it up with a starter).
For a typical 5 gallon batch of ale, you would be looking to make a 1 liter starter from a vial/package of liquid yeast. Double that for a lager.