Start with a list of what the trip requires. It helps identify the items you need to reach quickly and the space you actually need.
A bag inside the frame triangle suits heavier items such as tools or food. Use the manufacturer’s sizing template, if available, to check the fit against your frame. Protect mounting contact points from abrasion.
A saddle pack suits soft items such as spare clothing. Check saddle-rail and seatpost compatibility with its attachments; a dropper post may need a different solution. Follow that bag’s load limits and closure instructions.
Pack your intended luggage and try a short route before a longer journey. Convenient access to your things matters as much as capacity in litres.