There are two types the small ones that are the size of a paperback book but they tend to be about 200 amp capacity therefore not up to starting a V8.
The physically larger ones would be what you need for the V8.
Google for 500 amp or 600 amp jumpstart units to be sure of getting a V8 going.
This triggered my curiosity. I know that most starter motors are about 1000W so with a current of about 100 Amps since the voltage of a battery drops under that load. The starter circuit on my boat has a 120a trip which never trips so that hangs together. For a BMW V8 the figure I got from the net was 142 A.
In theory therefore then 200A would do. All other things being equal I would go for the bigger one cos bigger is always best!
For the benefit of the OP, avoid using such a gadget on a fully discharged battery with a modern CAN system.
