Ansom and his brothers meet in the center of it, indicating where the Hex barrier lies. we can grasp some idea of the minimum size of the hex, by looking at the layout of the Jetstone army. The bridge clearly does not have the width to it to make up the entire side of a hex. The hex has a part of it that is land, a part of it that is water, and maybe a part of it that has half a bridge over it. Depending on the rules these may all qualify as different terrain types and all are present within a single non-city hex.
It is possible that hex boundaries don't have to be exactly hexagons. The hex boundary could go down the middle of the river. The only requirement would be that they are roughly hexagons.