Actually 2 quarts is the right amount. This will allow the amount of yeast to roughly double in about 24 hours. those smack packs will only give you about half the amount of yeast you need to properly ferment a 5 gallon batch if you don't use a starter. Doing a quart of starter will not give you the growth you want.
A gallon jug will work just fine for the starter.
People do get by with using a quart, or a pint, or using none at all. That is a little like using a pair of pliers instead of a proper sized wrench. It will get the job done, but you may not like the results.
Wayne
Bugeater Brewing Company