This is a trade-off. You don't want to find that the pack you have just bought isn't big enough for your next trip, do you? However, this worry is a bit of an illusion. Consider a "large" 90 litre expedition pack: 90 litres of water weighs 90 kg. A large pack presents a serious risk of overload, simply because "there is room for it" and you aren't forced to seriously asked whether you "really need it". Further, most females should deliberately choose a slightly smaller pack because they have less upper-body strength than males. And what is male machismo for, anyhow?

A 60-70 litre pack should be quite enough for trips of a few days, up to a week in fact. But be careful that what you have chosen has a harness which fits you. You will find that the harnesses on some cheaper imported packs don't carry a load quite as well as good Australian and Kiwi packs. That is not a reason to reject an imported pack if it fits well: just keep the load down. I have a Tatonka which is fine up to 16 or maybe 18 kg, but I wouldn't want to put much more than that in it. A Macpac Cascade can carry over 25kg fairly easily - although you may have a lot of trouble standing up when it gets around 30kg. This classic pack puts a lot of the load fairly low, and this is good for climbing.

Many packs come in several sizes or back-lengths. I am not going to try to describe here what pack length you need for your back: that is best done by an experienced sales assistant in a good bushwalking shop. It is a moot point which is worse: too short or too long. Either way the big bum pad and waistband are not going to fit where they should, between the coccyx and the small of the back. Too long and the pack restricts the muscles in your backside; too short and the pack drags on your shoulders. Fortunately, good internal frame packs do have some length adjustment in the harness, and sales assistants should be trained in the adjustment. But it can still be worth while trying out small adjustments in the field yourself.