Two things about starting your car: you'll need more amps in cold weather than you will in warm; and, if your battery is really dead, you'll need even more power to get your engine restarted.
Get as many amps as you can reasonably afford. A smaller jump-starter will easily get your truck going on a warm day when the battery'* a little dead, but on a freezing-cold morning when your battery went dead weeks ago, you'll need significantly more juice to get it restarted.
I have a small portable jump-starter, but the battery in my G5 is barely bigger than a lawn mower'*.
Finally, general advice: don't forget to keep your jump-starter charged. Two dead batteries will get you about as far as one. Plan to keep it plugged in overnight at least once a month.