Most travellers end up using a rucksack and probably it won't be until you've undertaken a few trips that you actually can make a decision as to what the minimum size shape etc. is for you. Therefore if you can find some one to lend you one for your first trip it may well help you sort out in your mind exactly what you require.
If your pack may end up on a roof rack then take a bin liner to protect it and keep an eye on it at any stops. Take a cable lock so that on trains you can lock it to a luggage rack especially when your sleeping. It is also possible to get expandable wire cages that fit aound your ruck sack to stop light fingers at airports. However there have been some reports that these can wear into the rucksack material.
Rucksacks are rarely fully waterproof so either use a plastic liner or pack everything in plastic bags, don't forget the side and top pockets soggy maps and guide books can be difficult to dry out.
For low level walking except in winter a daysack of around 20 litres should be enough. However if you intend to walk in winter or on higher hills then a larger capacity of 25 to 35 litres is needed. For serious mountain walking you'll need 35-45 litres capacity for the extra clothing and equipment. This size pack is also useful for weekends away. If you are going to be carrying a heavy load check out the smaller backpacks as they tend to have more comfortable straps and padded hip belts. In general avoid daysacks whose main compartments close with zips, these tend to work their way open and if the zip breaks the pack is useless.
The most important thing about buying a rucksack is comfort. Its worth trying different manufactures and back systems. When trying them always ask the salesman to fill the pack so you have an idea what it feels like loaded. Make sure they fill it with something realistic. If they only put a non compressed sleeping bag in you'll have no idea of what it feels like loaded with clothing, food, tent, water etc. Ideally buy it from a shop where you can return it. Then take it home and fill it up with your kit. Try walking up and down stairs, bend down as if to pick something off the floor, then go for a short walk. It is vital the distance between the shoulder and strap and hip belt is correct so that the loaded is correctly distributed. Remember that if you fully load a backpack you've got to carry it.
First do up the waist strap tightly then adjust the shoulder straps so that the fit snuggly across your shoulders, the shoulder straps should not take the load off your hips. Most of the weight is there by transferred directly to your hips and legs and so there is limited downward force on your spine.
Ladies often find that as specifically designed lady racksack is better these packs tend to have shorter and anatomically designed back systems.
There are two schools of thought when choosing a size. One says choose a big sac you don't have to fill it and the other says choose a sac a size smaller than you think you need and through out all the excess crap you don't need. The author has travelled for several weeks with a backpack of 35 litres which held clothing and basic summer camping equipment.
|Up to 25||Smaller daysacks for walking except in winter conditions.|
|25 - 35||Winter day walking, summer lightweight camping, summer climbing, lightweight travelling|
|35 - 45||Serious winter walking, winter climbing, lightweight travelling.|
|55 - 75||Backpacking and travel rucksacks, most popular sizes.|
|75 -||Serious expedition use.|