Prewash must be at process-temp; the main purpose of it is to bring the tank to temp.
I can't speak to tank minima, but an 8x10 is basically one roll of film. Say you're doing E6 which has a typical capacity of ~10 rolls/L in two sequential batches (Fuji); you therefore want to make sure you have at least 200mL in the tank and run that working solution twice. Or if you can find a machine for the 2830 and run 2 of 8x10 at once, you want 400mL in the tank and run it twice. Those chemical-capacity minima are so much larger than the physical tank minima, so they're what you need to go by.
Though C41 has more capacity, I'd stick with the same numbers. Using up a small fraction of the chemical activity per process means you can make better guesses at the activity level in the tank, i.e. the developer is not noticeably slowing down within one development cycle.