Do you mean one of those portable jump packs? I would imagine how big depends on how dead you let the battery get.
We use them here at the shop to jump start dead cars all the time, but if the battery in the car is completely dead, sometimes we have to disconnect it before we attach the pack, otherwise the dead hole saps too much voltage trying to start the engine.
And this is just to get the car in the shop. The car's alternator should NEVER be used to charge up a dead battery. They won't live long that way, and the battery's life is substantially shortened, too.
If you are just concerned about an emergency backup, then I'd think in most cases you are not going to need too large of one.