The only thing that should be standard in a fat quarter is the length. It should be 18". Width will depend on the full width of the fabric. It will be 1/2 whatever the width of the fabric is - be it 39", 40", 41" etc.
I have seen different widths in the same design line of fabric. It just depends on where the particular fabric was printed. If it all was printed in the same plant then the chances are good that the width will be the same. Also even if printed in the same plant, there might be a slight difference based on where the base good came from.