You want to keep your starter in the range of 1.030 to 1.040 OG. That's roughly 10 oz of DME in 3 liters.
I use these jugs for my starters (the 1-gal version), which work out great. They cost something like $6 filled with apple juice or cider. They take about a size 8.5 stopper if you want to use an airlock.