It's not much larger than a typical smartphone, but the ToughTested Storm 8 Jump Starter will be a big help if you find yourself with a vehicle that won't start.
This 7x3" charger boats an output of 250 to 500 cranking amps. Its 8000mAh battery stays ready for up to one year, and can be recharged 1000 times. It comes with cables and alligator clips for attaching to the battery terminals. The built-in flashlight will help you find your way around under the hood at night, and the Storm 8 isn't affected by weather — it's rated IP65 water resistant.