Julian, looking at a number of strops, both new and vintage, they are in the neighborhood of 1/8" thick. Usually horsehide back in the old days. Cowhide works too. There is a member in Australia, IIRC, who has made strops of Kangaroo leather with good results. The default for the old pro barber strops was 2 1/2 wide x 24" long. In more recent times the 3" wide strop is more common. If you know what your preference is, one is probably as good as the other.