All I can tell you is don't skimp.
I have a PowerStation PSX whose power is described as follows: " High current EHP battery provides 1,000 Amps peak power, 400 Amps starting power'.
Had a dead battery out in the sticks and couldn't get the truck to turn over with this, however, I only tried for about a minute before I heard an engine in the distance and decided I'd better run and take advantage of my good fortune. I expect I would have eventually gotten restarted with more time but it takes more power to do so than I had assumed.