I had the same problem. I finally used the following workaround: 'hold on' the figure and use the 'quiver' command again in order to display just one vertical vector of a given length. Then write its length using e.g. text command. Mind the 'AutoScale' option, you better disable it in all 'quiver' commands in order to be sure about the vector length.
This solution requires that you have some empty space in your plot in order to display this extra vector and its length. Moreover, it does not display the reference vector in the legend but I think that this is ok.
