Really need to see a picture of it.
If it's ALICE, one size does not fit all, but it will last a lifetime, and the frames are still common and usually cheap.
If it's MOLLE, the early frames were easy to adjust for length, but they were pretty fragile, IMHO, hard to find, and expensive to replace. I haven't played with the latest, may be better than the early ones.
If it's a three day assualt pack, most of the common one have a plastic sheet for a stiffener in the back of the pack, and not much else.
