I was always told I had flat feet since I was young. As an adult the running shoe stores in Australia (like "Athlete's Foot") would always say I needed extra gel cushioning in both the heel and the forefoot.
After running in Vibram FiveFingers, as well as doing some truly barefoot running for a year or two, my feet did two interesting things. Firstly, my feet started to take up their natural space with the toes spreading out and getting back their independence and freedom. In doing so, my feet got a little longer and slightly wider. Secondly, my previously flat feet developed to have higher arches, which decreased my foot length by about 1/2 of a shoe size.
Between these two changes, my foot ended up back at its original size 13 in US size (or 46 in Vibram FiveFingers) but my feet were in much better form having been strengthened and developed through developing the good form that comes through being barefoot. My calf muscles and tendons strengthened gradually, but beware, doing too much barefoot activity too soon can lead to issues like tendonitis in the upper foot and other serious issues which can take many weeks or even months to recover from.
When you challenge your feet to develop with barefoot running/walking or minimalist footwear like Vibram FiveFingers, let your tired feet rest and recover for a day or two before challenging them again. It is imperative that you respect the training effect of overloading your feet with "reasonable" challenges followed by sufficient recovery before undertaking another "reasonable" challenge. Over time greater challenges become "reasonable" and as I did, you may find yourself able to run over 100 miles (162km) in 24 hours on sealed roads in Vibram FiveFingers with absolutely no blisters, no cramps and no biomechanical problems at all.