It depends on how long you're going and how you like to pack. As a general guide: under 25L works well for day trips and personal item slots on most airlines; 28–30L covers 1–3 day trips and fits under most seats; 38–40L handles 5–7 days carry-on without checking a bag; and 40L+ is suited for trips of a week or more.
Within this collection, the Blast Elite 22L and Faro 29L Set sit in the personal item range, the Blast 28L covers short trips, the Blast 38L and Faro 40L are the go-to for week-long carry-on travel, and the Blast Pro 40L is rated for up to two weeks. If you prefer to roll, the Blast 43L Roller covers the same carry-on range with the option to switch between backpack and wheeled mode. Check the individual product page for exact dimensions before booking, as airline limits vary by carrier.