There are numerous opinions about the proper size for a yeast starter. Formulas and calculators abound. I made an American Stout last Friday and pitched a starter I'd made two days before. I intended to make the beer on Thursday but with chores to do around the house I just couldn't get it done. I put the well fermenting (and quite large) starter in the fridge until I began the mash on Friday morning. I thought 4 liters of yeast might be a bit much for a 6 gallon batch but I pitched the whole thing for 3 reasons.
First, I made it from a 3 week old yeast cake I had saved from my latest Brown Ale. I have not begun washing and culturing yeast yet so I just left the spent hops in and mixed it up with a cup and a half of boiled dry malt.
Also, I didn't get it into the wort as it was at the height of fermentation. I chilled it for a day first.
Finally, this was a 1.072 beer. I wanted to err on the side of having more healthy yeast to help with the attenuation. I'd like this beer to finish out at 1.014 or less with a fairly clean flavor. The relatively high alcohol content will work against a low finish with California Ale yeast.
I have great confidence in Jamil Zainasheff's pitching rate calculator at http://www.mrmalty.com/ but I did it this way anyway. My only concern is the gravity change that a starter this size will do to the beer.
I'll be starting my study of yeast management soon.