The Golden Rule of backpacking is to keep your pack weight under 1/3 of your body weight. However, that’s actually higher than most backpackers would consider to be comfortable. Even following that “rule” for someone around 100 lbs, that’s less than 33 lbs which may be difficult for a longer trip unless you’re really interested in ultralight backpacking. For a small person, getting your backpack weight down can be extremely difficult since the weight your base gear (like your stove and tent) is not at all related to your body size. Let’s look at an example:
North Face Kazoo +20 Sleeping Bag
Short – Max Height 5’6″, Weight 2 lb 4 oz Regular – Max Height 6′, Weight 2 lb 7 oz Long – Max Height 6’6″, Weight 2 lb 9 oz
Extra Long – Max Height 7′, Weight 2 lb, 11 oz
You can see from the statistics above that the weight has very little variability between sleeping bag sizes, so a petite (short) bag will only be 2 oz less than a regular bag. If you’re traveling with a partner or group of people, you can figure out how to distribute weight so that everyone is comfortable. I won’t discuss this point now but if you are interested, there is also discussion that a smaller person might not have a disadvantage and the pack weight proportional to body weight rule might be a myth (Read HERE).
> For reference, I am 5’1″ and ~100 lbs