Here are a few metrics:
maximum cell density for 1ml = ~200 million
maximum cell density for 500ml = 500 * 200 million = ~100 billion
maximum cell density for 1L = 1,000 * 200 million = ~200 billion
maximum cell density for one U.S. gallon = 3,785 * 200 million = ~757 billion
The fact that the starter volume should be no more than 1/4th of the vessel volume when making a starter using my method does not impact these values.
Your standard starter size should be 200ml, which gives you the same 1:19 pitching ratio as a brewer pitching a 1L starter into 5 gallons of wort.
maximum cell density for 200ml = 200 * 200 million = 40 billion cells
To keep the same ratio as someone pitching a culture that started out as 100 billion cells into 1L, you going to have to pitch 1/5th of a 100 billion cell package into 200ml. In my humble opinion, it's not worth making a starter with one gallon batches unless you are starting from slant. Small batches are a bit of a logistics problem when using commercial yeast. Half of a Wyeast Activator pack will be enough active yeast to fully attenuate your batch. I would make two one gallon batches, split the package between them, and harvest slurry.
Collecting slurry from a one gallon batch can be a bit of a problem because one gallon does not leave you with much liquid after racking, but you can do it. You need to collect between 17ml and 33ml of thick slurry to repitch a batch. That amount of slurry will give you between 20 and 40 billion cells, which is equivalent to collecting 84ml to 168 milliliters of thick slurry for a 5-gallon batch. Due to exponential growth, you do not need to worry about being exactly on the money (i.e., yeast cultures are like nuclear weapons in that close is good enough); however, you are better off collecting at the higher end than the lower end because not all of the cells are viable.