On most airlines, including major US airlines like United, Delta, and American, the maximum size for carry on luggage is 22" x 14" x 9". Tortuga's travel backpacks and carry on duffles always conform to those requirements. Note that the Outbreaker 45L backpack and the Homebase duffle case are both maximum size carry ons.
Some airlines, particularly those with budget fares, are more restrictive in regards to what qualifies as a carry on. If you frequently fly on budget airlines like RyanAir or EasyJet, you may want to buy a mid-size carry on like the Outbreaker 35L backpack or the Homebase backpack.
Make sure to check with your favorite airline to confirm their specific carry on rules.