A wall to overcome

As I learning how to create this Foot measuring socks i come across a problem, I was intended the socks to be flexible like normal socks to be able to put on by any size foot but I don't know how to make them measure the length, width and height of the foot while socks being flexible. While looking for any method for measuring three dimensional I didn't come across devices that are flexible and small enough to put in a sock, unless I find a way to incorporate measuring system without increasing size of the socks it would end up being heavy and inflexible which in turn will not fit for all foot size to measure. I am looking into this but the technology is not good enough or end up being more expensive to buy.