Here are some things to consider when determining what size bag to get:

Consider the length and type of trip you’re taking. Are you going on a serious adventure that requires tents, sleeping bags, stoves, climbing gear and more? If so, you’re going to need a large bag (or several) to haul all that. But, if you’re going on a simple weekend getaway that requires a couple changes of clothes and toiletries, then a compact bagpack will work just fine.